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Date: Tue, 8 Dec 2015 00:46:01 +0800
From: Gavin Guo <gavin.guo@...onical.com>
To: Dave Chinner <david@...morbit.com>
Cc: xfs@....sgi.com, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Possible memory allocation deadlock in kmem_alloc and hung task
in xfs_log_commit_cil and xlog_cil_push
Hi Dave,
On Mon, Aug 31, 2015 at 3:33 PM, Gavin Guo <gavin.guo@...onical.com> wrote:
> On Sun, Aug 30, 2015 at 6:31 AM, Dave Chinner <david@...morbit.com> wrote:
>> On Fri, Aug 28, 2015 at 08:54:04PM +0800, Gavin Guo wrote:
>>> On Wed, Jul 8, 2015 at 7:37 AM, Dave Chinner <david@...morbit.com> wrote:
>>> > On Tue, Jul 07, 2015 at 05:29:43PM +0800, Gavin Guo wrote:
>>> >> Hi all,
>>> >>
>>> >> Recently, we observed that there is the error message in
>>> >> Ubuntu-3.13.0-48.80:
>>> >>
>>> >> "XFS: possible memory allocation deadlock in kmem_alloc (mode:0x8250)"
>>> >>
>>> >> repeatedly shows in the dmesg. Temporarily, our workaround is to tune the
>>> >> parameters, such as, vfs_cache_pressure, min_free_kbytes, and dirty_ratio.
>>> >>
>>> >> And we also found that there are different error messages regarding the
>>> >> hung tasks which happened in xfs_log_commit_cil and xlog_cil_push.
>>> >>
>>> >> The log is available at: http://paste.ubuntu.com/11835007/
>>> >>
>>> >> The following link seems the same problem we suffered:
>>> >>
>>> >> XFS hangs with XFS: possible memory allocation deadlock in kmem_alloc
>>> >> http://oss.sgi.com/archives/xfs/2015-03/msg00172.html
>>> >>
>>> >> I read the mail and found that there might be some modification regarding
>>> >> to move the memory allocation outside the ctx lock. And I also read the
>>> >> latest patch from February of 2015 to see if there is any new change
>>> >> about that. Unfortunately, I didn't find anything regarding the change (may
>>> >> be I'm not familiar with the XFS, so didn't find the commit). If it's
>>> >> possible for someone who is familiar with the code to point out the commits
>>> >> related to the bug if already exist or any status about the plan.
>>> >
>>> > No commits - the approach I thought we might be able to take to
>>> > avoid the problem didn't work out. I have another idea of how we
>>> > might solve the problem, but I haven't ad a chance to prototype it
>>> > yet.
>>>
>>> I have read the code for a while and still can't figure out how to fix.
>>> My current understanding is that the problem is Buddy system is running out
>>> of memory so the XFS kmem_alloc(),
>>>
>>> called by xfs_log_commit_cil->
>>> xlog_cil_insert_items->
>>> xlog_cil_insert_format_items->
>>> kmem_zalloc,
>>>
>>> fail and stuck in the while loop and retry. There are also 2 other threads
>>> running in the same time:
>>>
>>> 1). xfs_log_commit_cil->down_read(&cil->xc_ctx_lock);
>>>
>>> 2). xlog_cil_push->down_write(&cil->xc_ctx_lock);
>>>
>>> So, the both threads are blocked and waiting for the first kmem_zalloc() to
>>> succeed.
>>>
>>> However, if there is a way to decrease the memory request or if it's
>>> possible to elaborate more on the idea you mentioned. I know it's a
>>> problem which cannot be solved in a short time. And I'd like to help if
>>> there is any possibility.
>>
>> This is the patch I'm currently working on. It doesn't work
>> completely yet, but it will give you an idea of how the problem
>> needs tobe solved (read the big comment in the patch).
>>
>> -Dave.
>> --
>> Dave Chinner
>> david@...morbit.com
>>
>> ---
>> fs/xfs/xfs_buf_item.c | 1 +
>> fs/xfs/xfs_dquot.c | 1 +
>> fs/xfs/xfs_dquot_item.c | 2 +
>> fs/xfs/xfs_extfree_item.c | 2 +
>> fs/xfs/xfs_inode_item.c | 1 +
>> fs/xfs/xfs_log_cil.c | 228 ++++++++++++++++++++++++++++++++++------------
>> 6 files changed, 177 insertions(+), 58 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_buf_item.c b/fs/xfs/xfs_buf_item.c
>> index 1816334..64cd236 100644
>> --- a/fs/xfs/xfs_buf_item.c
>> +++ b/fs/xfs/xfs_buf_item.c
>> @@ -831,6 +831,7 @@ xfs_buf_item_free(
>> xfs_buf_log_item_t *bip)
>> {
>> xfs_buf_item_free_format(bip);
>> + kmem_free(bip->bli_item->li_lv_shadow);
>> kmem_zone_free(xfs_buf_item_zone, bip);
>> }
>>
>> diff --git a/fs/xfs/xfs_dquot.c b/fs/xfs/xfs_dquot.c
>> index 4143dc7..b9e7dda 100644
>> --- a/fs/xfs/xfs_dquot.c
>> +++ b/fs/xfs/xfs_dquot.c
>> @@ -74,6 +74,7 @@ xfs_qm_dqdestroy(
>> {
>> ASSERT(list_empty(&dqp->q_lru));
>>
>> + kmem_free(dqp->qli_item.li_lv_shadow);
>> mutex_destroy(&dqp->q_qlock);
>> kmem_zone_free(xfs_qm_dqzone, dqp);
>>
>> diff --git a/fs/xfs/xfs_dquot_item.c b/fs/xfs/xfs_dquot_item.c
>> index 814cff9..2c7a162 100644
>> --- a/fs/xfs/xfs_dquot_item.c
>> +++ b/fs/xfs/xfs_dquot_item.c
>> @@ -370,6 +370,8 @@ xfs_qm_qoffend_logitem_committed(
>> spin_lock(&ailp->xa_lock);
>> xfs_trans_ail_delete(ailp, &qfs->qql_item, SHUTDOWN_LOG_IO_ERROR);
>>
>> + kmem_free(qfs->qql_item.li_lv_shadow);
>> + kmem_free(lip->li_lv_shadow);
>> kmem_free(qfs);
>> kmem_free(qfe);
>> return (xfs_lsn_t)-1;
>> diff --git a/fs/xfs/xfs_extfree_item.c b/fs/xfs/xfs_extfree_item.c
>> index adc8f8f..3842418 100644
>> --- a/fs/xfs/xfs_extfree_item.c
>> +++ b/fs/xfs/xfs_extfree_item.c
>> @@ -40,6 +40,7 @@ void
>> xfs_efi_item_free(
>> struct xfs_efi_log_item *efip)
>> {
>> + kmem_free(efip->efi_item.li_lv_shadow);
>> if (efip->efi_format.efi_nextents > XFS_EFI_MAX_FAST_EXTENTS)
>> kmem_free(efip);
>> else
>> @@ -329,6 +330,7 @@ static inline struct xfs_efd_log_item *EFD_ITEM(struct xfs_log_item *lip)
>> STATIC void
>> xfs_efd_item_free(struct xfs_efd_log_item *efdp)
>> {
>> + kmem_free(efdp->efd_item.li_lv_shadow);
>> if (efdp->efd_format.efd_nextents > XFS_EFD_MAX_FAST_EXTENTS)
>> kmem_free(efdp);
>> else
>> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
>> index bf13a5a..39ca237 100644
>> --- a/fs/xfs/xfs_inode_item.c
>> +++ b/fs/xfs/xfs_inode_item.c
>> @@ -577,6 +577,7 @@ void
>> xfs_inode_item_destroy(
>> xfs_inode_t *ip)
>> {
>> + kmem_free(ip->ili_item->li_lv_shadow);
>> kmem_zone_free(xfs_ili_zone, ip->i_itemp);
>> }
>>
>> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
>> index abc2ccb..ab4b98c 100644
>> --- a/fs/xfs/xfs_log_cil.c
>> +++ b/fs/xfs/xfs_log_cil.c
>> @@ -79,6 +79,145 @@ xlog_cil_init_post_recovery(
>> log->l_cilp->xc_ctx->sequence = 1;
>> }
>>
>> +static inline int
>> +xlog_cil_iovec_space(
>> + uint niovecs)
>> +{
>> + return round_up((sizeof(struct xfs_log_vec) +
>> + niovecs * sizeof(struct xfs_log_iovec)),
>> + sizeof(uint64_t));
>> +}
>> +
>> +/*
>> + * Allocate or pin log vector buffers for CIL insertion.
>> + *
>> + * The CIL currently uses disposable buffers for copying a snapshot of the
>> + * modified items into the log during a push. The biggest problem with this is
>> + * the requirement to allocate the disposable buffer during the commit if:
>> + * a) does not exist; or
>> + * b) it is too small
>> + *
>> + * If we do this allocation within xlog_cil_insert_format_items(), it is done
>> + * under the xc_ctx_lock, which means that a CIL push cannot occur during
>> + * the memory allocation. This means that we have a potential deadlock situation
>> + * under low memory conditions when we have lots of dirty metadata pinned in
>> + * the CIL and we need a CIL commit to occur to free memory.
>> + *
>> + * To avoid this, we need to move the memory allocation outside the
>> + * xc_ctx_lock(), but because the log vector buffers are disposable, that opens
>> + * up a TOCTOU race condition w.r.t. the CIL commiting and removing the log
>> + * vector buffers between the check and the formatting of the item into the
>> + * log vector buffer within the xc_ctx_lock.
>> + *
>> + * Because the log vector buffer needs to be unchanged during the CIL push
>> + * process, we cannot share the buffer between the transaction commit (which
>> + * modifies the buffer) and the CIL push context that is writing the changes
>> + * into the log. This means skipping preallocation of buffer space is
>> + * unreliable, but we most definitely do not want to be allocating and freeing
>> + * buffers unnecessarily during commits when overwrites can be done safely.
>> + *
>> + * The simplest solution to this problem is to allocate a shadow buffer when a
>> + * log item is committed for the second time, and then to only use this buffer
>> + * if necessary. The buffer can remain attached to the log item until such time
>> + * it is needed, and this is the buffer that is reallocated to match the size of
>> + * the incoming modification. Then during the formatting of the item we can swap
>> + * the active buffer with the new one if we can't reuse the existing buffer. We
>> + * don't free the old buffer as it may be reused on the next modification if
>> + * it's size is right, otherwise we'll free and reallocate it at that point.
>> + *
>> + * This function builds a vector for the changes in each log item in the
>> + * transaction. It then works out the length of the buffer needed for each log
>> + * item, allocates them and attaches the vector to the log item in preparation
>> + * for the formatting step which occurs under the xc_ctx_lock.
>> + *
>> + * While this means the memory footprint goes up, it avoids the repeated
>> + * alloc/free pattern that repeated modifications of an item would otherwise
>> + * cause, and hence minimises the CPU overhead of such behaviour.
>> + */
>> +static void
>> +xfs_cil_item_alloc_shadow_lvbufs(
>> + struct xlog *log,
>> + struct xfs_trans *tp)
>> +{
>> + list_for_each_entry(lidp, &tp->t_items, lid_trans) {
>> + struct xfs_log_item *lip = lidp->lid_item;
>> + struct xfs_log_vec *lv;
>> + struct xfs_log_vec *old_lv;
>> + int niovecs = 0;
>> + int nbytes = 0;
>> + int buf_size;
>> + bool ordered = false;
>> +
>> + /* Skip items which aren't dirty in this transaction. */
>> + if (!(lidp->lid_flags & XFS_LID_DIRTY))
>> + continue;
>> +
>> + /* get number of vecs and size of data to be stored */
>> + lip->li_ops->iop_size(lip, &niovecs, &nbytes);
>> +
>> + /*
>> + * Ordered items need to be tracked but we do not wish to write
>> + * them. We need a logvec to track the object, but we do not
>> + * need an iovec or buffer to be allocated for copying data.
>> + */
>> + if (niovecs == XFS_LOG_VEC_ORDERED) {
>> + ordered = true;
>> + niovecs = 0;
>> + nbytes = 0;
>> + }
>> +
>> + /*
>> + * We 64-bit align the length of each iovec so that the start
>> + * of the next one is naturally aligned. We'll need to
>> + * account for that slack space here. Then round nbytes up
>> + * to 64-bit alignment so that the initial buffer alignment is
>> + * easy to calculate and verify.
>> + */
>> + nbytes += niovecs * sizeof(uint64_t);
>> + nbytes = round_up(nbytes, sizeof(uint64_t));
>> +
>> + /* grab the old item if it exists for reservation accounting */
>> + old_lv = lip->li_lv;
>> +
>> + /*
>> + * The data buffer needs to start 64-bit aligned, so round up
>> + * that space to ensure we can align it appropriately and not
>> + * overrun the buffer.
>> + */
>> + buf_size = nbytes + xlog_cil_iovec_space(niovecs);
>> +
>> + /*
>> + * if we have no shadow buffer, or it is too small, we need to
>> + * reallocate it.
>> + */
>> + if (!lip->li_lv_shadow ||
>> + buf_size <= lip->li_lv_shadow->lv_size) {
>> +
>> + kmem_free(lip->li_lv_shadow);
>> +
>> + lv = kmem_zalloc(buf_size, KM_SLEEP|KM_NOFS);
>> + lv->lv_item = lip;
>> + lv->lv_size = buf_size;
>> + if (ordered)
>> + lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
>> + else
>> + lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
>> + lip->li_lv_shadow = lv;
>> + } else {
>> + /* same or smaller, optimise common overwrite case */
>> + lv = lip->li_lv_shadow;
>> + }
>> +
>> + /* Ensure the lv is set up according to ->iop_size */
>> + lv->lv_niovecs = niovecs;
>> +
>> + /* The allocated data region lies beyond the iovec region */
>> + lv->lv_buf_len = 0;
>> + lv->lv_bytes = 0;
>> + lv->lv_buf = (char *)lv + xlog_cil_iovec_space(niovecs);
>> + }
>> +
>> +}
>> /*
>> * Prepare the log item for insertion into the CIL. Calculate the difference in
>> * log space and vectors it will consume, and if it is a new item pin it as
>> @@ -101,7 +240,8 @@ xfs_cil_prepare_item(
>> /*
>> * If there is no old LV, this is the first time we've seen the item in
>> * this CIL context and so we need to pin it. If we are replacing the
>> - * old_lv, then remove the space it accounts for and free it.
>> + * old_lv, then remove the space it accounts for and make it the shadow
>> + * buffer for later freeing.
>> */
>> if (!old_lv)
>> lv->lv_item->li_ops->iop_pin(lv->lv_item);
>> @@ -110,7 +250,7 @@ xfs_cil_prepare_item(
>>
>> *diff_len -= old_lv->lv_bytes;
>> *diff_iovecs -= old_lv->lv_niovecs;
>> - kmem_free(old_lv);
>> + lip->li_lv_shadow = old_lv;
>> }
>>
>> /* attach new log vector to log item */
>> @@ -134,11 +274,13 @@ xfs_cil_prepare_item(
>> * write it out asynchronously without needing to relock the object that was
>> * modified at the time it gets written into the iclog.
>> *
>> - * This function builds a vector for the changes in each log item in the
>> - * transaction. It then works out the length of the buffer needed for each log
>> - * item, allocates them and formats the vector for the item into the buffer.
>> - * The buffer is then attached to the log item are then inserted into the
>> - * Committed Item List for tracking until the next checkpoint is written out.
>> + * This function takes the prepared log vectors attached to each log item, and
>> + * formats the changes into the log vector buffer. The buffer it uses is
>> + * dependent on the current state of the vector in the CIL - the shadow lv is
>> + * guaranteed to be large enough for the current modification, but we will only
>> + * use that if we can't reuse the existing lv. If we can't reuse the existing
>> + * lv, then simple swap it out for the shadow lv. We don't free it - that is
>> + * done lazily either by th enext modification or the freeing of the log item.
>> *
>> * We don't set up region headers during this process; we simply copy the
>> * regions into the flat buffer. We can do this because we still have to do a
>> @@ -171,7 +313,8 @@ xlog_cil_insert_format_items(
>> list_for_each_entry(lidp, &tp->t_items, lid_trans) {
>> struct xfs_log_item *lip = lidp->lid_item;
>> struct xfs_log_vec *lv;
>> - struct xfs_log_vec *old_lv;
>> + struct xfs_log_vec *old_lv = NULL;
>> + struct xfs_log_vec *shadow;
>> int niovecs = 0;
>> int nbytes = 0;
>> int buf_size;
>> @@ -181,49 +324,19 @@ xlog_cil_insert_format_items(
>> if (!(lidp->lid_flags & XFS_LID_DIRTY))
>> continue;
>>
>> - /* get number of vecs and size of data to be stored */
>> - lip->li_ops->iop_size(lip, &niovecs, &nbytes);
>> -
>> - /* Skip items that do not have any vectors for writing */
>> - if (!niovecs)
>> - continue;
>> -
>> /*
>> - * Ordered items need to be tracked but we do not wish to write
>> - * them. We need a logvec to track the object, but we do not
>> - * need an iovec or buffer to be allocated for copying data.
>> + * The formatting size information is already attached to
>> + * the shadow lv on the log item.
>> */
>> - if (niovecs == XFS_LOG_VEC_ORDERED) {
>> + if (shadow->lv_buf_len == XFS_LOG_VEC_ORDERED)
>> ordered = true;
>> - niovecs = 0;
>> - nbytes = 0;
>> - }
>>
>> - /*
>> - * We 64-bit align the length of each iovec so that the start
>> - * of the next one is naturally aligned. We'll need to
>> - * account for that slack space here. Then round nbytes up
>> - * to 64-bit alignment so that the initial buffer alignment is
>> - * easy to calculate and verify.
>> - */
>> - nbytes += niovecs * sizeof(uint64_t);
>> - nbytes = round_up(nbytes, sizeof(uint64_t));
>> -
>> - /* grab the old item if it exists for reservation accounting */
>> - old_lv = lip->li_lv;
>> -
>> - /*
>> - * The data buffer needs to start 64-bit aligned, so round up
>> - * that space to ensure we can align it appropriately and not
>> - * overrun the buffer.
>> - */
>> - buf_size = nbytes +
>> - round_up((sizeof(struct xfs_log_vec) +
>> - niovecs * sizeof(struct xfs_log_iovec)),
>> - sizeof(uint64_t));
>> + /* Skip items that do not have any vectors for writing */
>> + if (!shadow->lv_niovecs && !ordered)
>> + continue;
>>
>> /* compare to existing item size */
>> - if (lip->li_lv && buf_size <= lip->li_lv->lv_size) {
>> + if (lip->li_lv && shadow->lv_size <= lip->li_lv->lv_size) {
>> /* same or smaller, optimise common overwrite case */
>> lv = lip->li_lv;
>> lv->lv_next = NULL;
>> @@ -237,29 +350,28 @@ xlog_cil_insert_format_items(
>> */
>> *diff_iovecs -= lv->lv_niovecs;
>> *diff_len -= lv->lv_bytes;
>> +
>> + /* Ensure the lv is set up according to ->iop_size */
>> + lv->lv_niovecs = shadow->lv_niovecs;
>> +
>> + /* reset the lv buffer information for new formatting */
>> + lv->lv_buf_len = 0;
>> + lv->lv_bytes = 0;
>> + lv->lv_buf = (char *)lv +
>> + xlog_cil_iovec_space(lv->lv_niovecs)
>> } else {
>> - /* allocate new data chunk */
>> - lv = kmem_zalloc(buf_size, KM_SLEEP|KM_NOFS);
>> + /* switch to shadow buffer! */
>> + lv = shadow;
>> lv->lv_item = lip;
>> - lv->lv_size = buf_size;
>> + old_lv = lip->li_lv;
>> if (ordered) {
>> /* track as an ordered logvec */
>> ASSERT(lip->li_lv == NULL);
>> - lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
>> goto insert;
>> }
>> - lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
>> }
>>
>> - /* Ensure the lv is set up according to ->iop_size */
>> - lv->lv_niovecs = niovecs;
>> -
>> - /* The allocated data region lies beyond the iovec region */
>> - lv->lv_buf_len = 0;
>> - lv->lv_bytes = 0;
>> - lv->lv_buf = (char *)lv + buf_size - nbytes;
>> ASSERT(IS_ALIGNED((unsigned long)lv->lv_buf, sizeof(uint64_t)));
>> -
>> lip->li_ops->iop_format(lip, lv);
>> insert:
>> ASSERT(lv->lv_buf_len <= nbytes);
>
> Really thanks for your patch. I'll study the patch first to figure out the idea.
>
> Gavin
I'm keeping on watching the mailing list and seems that the patch is still
under development. Just want to confirm I didn't miss any information and
I am more than happy to give any help if needed. :)
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