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Message-ID: <0a383faf-40dc-4ea4-9a05-dbbb598f2184@huawei.com>
Date: Mon, 1 Jul 2024 09:21:18 +0800
From: Zizhi Wo <wozizhi@...wei.com>
To: <chandan.babu@...cle.com>, <djwong@...nel.org>, <dchinner@...hat.com>
CC: <linux-xfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
<yangerkun@...wei.com>
Subject: Re: [PATCH V3] xfs: Avoid races with cnt_btree lastrec updates
friendly ping
在 2024/6/25 9:46, Zizhi Wo 写道:
> A concurrent file creation and little writing could unexpectedly return
> -ENOSPC error since there is a race window that the allocator could get
> the wrong agf->agf_longest.
>
> Write file process steps:
> 1) Find the entry that best meets the conditions, then calculate the start
> address and length of the remaining part of the entry after allocation.
> 2) Delete this entry and update the -current- agf->agf_longest.
> 3) Insert the remaining unused parts of this entry based on the
> calculations in 1), and update the agf->agf_longest again if necessary.
>
> Create file process steps:
> 1) Check whether there are free inodes in the inode chunk.
> 2) If there is no free inode, check whether there has space for creating
> inode chunks, perform the no-lock judgment first.
> 3) If the judgment succeeds, the judgment is performed again with agf lock
> held. Otherwire, an error is returned directly.
>
> If the write process is in step 2) but not go to 3) yet, the create file
> process goes to 2) at this time, it may be mistaken for no space,
> resulting in the file system still has space but the file creation fails.
>
> We have sent two different commits to the community in order to fix this
> problem[1][2]. Unfortunately, both solutions have flaws. In [2], I
> discussed with Dave and Darrick, realized that a better solution to this
> problem requires the "last cnt record tracking" to be ripped out of the
> generic btree code. And surprisingly, Dave directly provided his fix code.
> This patch includes appropriate modifications based on his tmp-code to
> address this issue.
>
> The entire fix can be roughly divided into two parts:
> 1) Delete the code related to lastrec-update in the generic btree code.
> 2) Place the process of updating longest freespace with cntbt separately
> to the end of the cntbt modifications. Move the cursor to the rightmost
> firstly, and update the longest free extent based on the record.
>
> Note that we can not update the longest with xfs_alloc_get_rec() after
> find the longest record, as xfs_verify_agbno() may not pass because
> pag->block_count is updated on the outside. Therefore, use
> xfs_btree_get_rec() as a replacement.
>
> [1] https://lore.kernel.org/all/20240419061848.1032366-2-yebin10@huawei.com
> [2] https://lore.kernel.org/all/20240604071121.3981686-1-wozizhi@huawei.com
>
> Reported by: Ye Bin <yebin10@...wei.com>
> Signed-off-by: Zizhi Wo <wozizhi@...wei.com>
> ---
> fs/xfs/libxfs/xfs_alloc.c | 115 ++++++++++++++++++++++++++++++++
> fs/xfs/libxfs/xfs_alloc_btree.c | 64 ------------------
> fs/xfs/libxfs/xfs_btree.c | 51 --------------
> fs/xfs/libxfs/xfs_btree.h | 16 +----
> 4 files changed, 116 insertions(+), 130 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 6c55a6e88eba..ef8415af47f8 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -465,6 +465,98 @@ xfs_alloc_fix_len(
> args->len = rlen;
> }
>
> +/*
> + * Determine if the cursor points to the block that contains the right-most
> + * block of records in the by-count btree. This block contains the largest
> + * contiguous free extent in the AG, so if we modify a record in this block we
> + * need to call xfs_alloc_fixup_longest() once the modifications are done to
> + * ensure the agf->agf_longest field is kept up to date with the longest free
> + * extent tracked by the by-count btree.
> + */
> +static bool
> +xfs_alloc_cursor_at_lastrec(
> + struct xfs_btree_cur *cnt_cur)
> +{
> + struct xfs_btree_block *block;
> + union xfs_btree_ptr ptr;
> + struct xfs_buf *bp;
> +
> + block = xfs_btree_get_block(cnt_cur, 0, &bp);
> +
> + xfs_btree_get_sibling(cnt_cur, block, &ptr, XFS_BB_RIGHTSIB);
> + return xfs_btree_ptr_is_null(cnt_cur, &ptr);
> +}
> +
> +/*
> + * Find the rightmost record of the cntbt, and return the longest free space
> + * recorded in it. Simply set both the block number and the length to their
> + * maximum values before searching.
> + */
> +static int
> +xfs_cntbt_longest(
> + struct xfs_btree_cur *cnt_cur,
> + xfs_extlen_t *longest)
> +{
> + struct xfs_alloc_rec_incore irec;
> + union xfs_btree_rec *rec;
> + int stat = 0;
> + int error;
> +
> + memset(&cnt_cur->bc_rec, 0xFF, sizeof(cnt_cur->bc_rec));
> + error = xfs_btree_lookup(cnt_cur, XFS_LOOKUP_LE, &stat);
> + if (error)
> + return error;
> + if (!stat) {
> + /* totally empty tree */
> + *longest = 0;
> + return 0;
> + }
> +
> + error = xfs_btree_get_rec(cnt_cur, &rec, &stat);
> + if (error)
> + return error;
> + if (!stat) {
> + ASSERT(0);
> + *longest = 0;
> + return 0;
> + }
> +
> + xfs_alloc_btrec_to_irec(rec, &irec);
> + *longest = irec.ar_blockcount;
> + return 0;
> +}
> +
> +/*
> + * Update the longest contiguous free extent in the AG from the by-count cursor
> + * that is passed to us. This should be done at the end of any allocation or
> + * freeing operation that touches the longest extent in the btree.
> + *
> + * Needing to update the longest extent can be determined by calling
> + * xfs_alloc_cursor_at_lastrec() after the cursor is positioned for record
> + * modification but before the modification begins.
> + */
> +static int
> +xfs_alloc_fixup_longest(
> + struct xfs_btree_cur *cnt_cur)
> +{
> + struct xfs_perag *pag = cnt_cur->bc_ag.pag;
> + struct xfs_buf *bp = cnt_cur->bc_ag.agbp;
> + struct xfs_agf *agf = bp->b_addr;
> + xfs_extlen_t longest = 0;
> + int error;
> +
> + /* Lookup last rec in order to update AGF. */
> + error = xfs_cntbt_longest(cnt_cur, &longest);
> + if (error)
> + return error;
> +
> + pag->pagf_longest = longest;
> + agf->agf_longest = cpu_to_be32(pag->pagf_longest);
> + xfs_alloc_log_agf(cnt_cur->bc_tp, bp, XFS_AGF_LONGEST);
> +
> + return 0;
> +}
> +
> /*
> * Update the two btrees, logically removing from freespace the extent
> * starting at rbno, rlen blocks. The extent is contained within the
> @@ -489,6 +581,7 @@ xfs_alloc_fixup_trees(
> xfs_extlen_t nflen1=0; /* first new free length */
> xfs_extlen_t nflen2=0; /* second new free length */
> struct xfs_mount *mp;
> + bool fixup_longest = false;
>
> mp = cnt_cur->bc_mp;
>
> @@ -577,6 +670,10 @@ xfs_alloc_fixup_trees(
> nfbno2 = rbno + rlen;
> nflen2 = (fbno + flen) - nfbno2;
> }
> +
> + if (xfs_alloc_cursor_at_lastrec(cnt_cur))
> + fixup_longest = true;
> +
> /*
> * Delete the entry from the by-size btree.
> */
> @@ -654,6 +751,10 @@ xfs_alloc_fixup_trees(
> return -EFSCORRUPTED;
> }
> }
> +
> + if (fixup_longest)
> + return xfs_alloc_fixup_longest(cnt_cur);
> +
> return 0;
> }
>
> @@ -1956,6 +2057,7 @@ xfs_free_ag_extent(
> int i;
> int error;
> struct xfs_perag *pag = agbp->b_pag;
> + bool fixup_longest = false;
>
> bno_cur = cnt_cur = NULL;
> mp = tp->t_mountp;
> @@ -2219,8 +2321,13 @@ xfs_free_ag_extent(
> }
> xfs_btree_del_cursor(bno_cur, XFS_BTREE_NOERROR);
> bno_cur = NULL;
> +
> /*
> * In all cases we need to insert the new freespace in the by-size tree.
> + *
> + * If this new freespace is being inserted in the block that contains
> + * the largest free space in the btree, make sure we also fix up the
> + * agf->agf-longest tracker field.
> */
> if ((error = xfs_alloc_lookup_eq(cnt_cur, nbno, nlen, &i)))
> goto error0;
> @@ -2229,6 +2336,8 @@ xfs_free_ag_extent(
> error = -EFSCORRUPTED;
> goto error0;
> }
> + if (xfs_alloc_cursor_at_lastrec(cnt_cur))
> + fixup_longest = true;
> if ((error = xfs_btree_insert(cnt_cur, &i)))
> goto error0;
> if (XFS_IS_CORRUPT(mp, i != 1)) {
> @@ -2236,6 +2345,12 @@ xfs_free_ag_extent(
> error = -EFSCORRUPTED;
> goto error0;
> }
> + if (fixup_longest) {
> + error = xfs_alloc_fixup_longest(cnt_cur);
> + if (error)
> + goto error0;
> + }
> +
> xfs_btree_del_cursor(cnt_cur, XFS_BTREE_NOERROR);
> cnt_cur = NULL;
>
> diff --git a/fs/xfs/libxfs/xfs_alloc_btree.c b/fs/xfs/libxfs/xfs_alloc_btree.c
> index 6ef5ddd89600..585e98e87ef9 100644
> --- a/fs/xfs/libxfs/xfs_alloc_btree.c
> +++ b/fs/xfs/libxfs/xfs_alloc_btree.c
> @@ -115,67 +115,6 @@ xfs_allocbt_free_block(
> return 0;
> }
>
> -/*
> - * Update the longest extent in the AGF
> - */
> -STATIC void
> -xfs_allocbt_update_lastrec(
> - struct xfs_btree_cur *cur,
> - const struct xfs_btree_block *block,
> - const union xfs_btree_rec *rec,
> - int ptr,
> - int reason)
> -{
> - struct xfs_agf *agf = cur->bc_ag.agbp->b_addr;
> - struct xfs_perag *pag;
> - __be32 len;
> - int numrecs;
> -
> - ASSERT(!xfs_btree_is_bno(cur->bc_ops));
> -
> - switch (reason) {
> - case LASTREC_UPDATE:
> - /*
> - * If this is the last leaf block and it's the last record,
> - * then update the size of the longest extent in the AG.
> - */
> - if (ptr != xfs_btree_get_numrecs(block))
> - return;
> - len = rec->alloc.ar_blockcount;
> - break;
> - case LASTREC_INSREC:
> - if (be32_to_cpu(rec->alloc.ar_blockcount) <=
> - be32_to_cpu(agf->agf_longest))
> - return;
> - len = rec->alloc.ar_blockcount;
> - break;
> - case LASTREC_DELREC:
> - numrecs = xfs_btree_get_numrecs(block);
> - if (ptr <= numrecs)
> - return;
> - ASSERT(ptr == numrecs + 1);
> -
> - if (numrecs) {
> - xfs_alloc_rec_t *rrp;
> -
> - rrp = XFS_ALLOC_REC_ADDR(cur->bc_mp, block, numrecs);
> - len = rrp->ar_blockcount;
> - } else {
> - len = 0;
> - }
> -
> - break;
> - default:
> - ASSERT(0);
> - return;
> - }
> -
> - agf->agf_longest = len;
> - pag = cur->bc_ag.agbp->b_pag;
> - pag->pagf_longest = be32_to_cpu(len);
> - xfs_alloc_log_agf(cur->bc_tp, cur->bc_ag.agbp, XFS_AGF_LONGEST);
> -}
> -
> STATIC int
> xfs_allocbt_get_minrecs(
> struct xfs_btree_cur *cur,
> @@ -493,7 +432,6 @@ const struct xfs_btree_ops xfs_bnobt_ops = {
> .set_root = xfs_allocbt_set_root,
> .alloc_block = xfs_allocbt_alloc_block,
> .free_block = xfs_allocbt_free_block,
> - .update_lastrec = xfs_allocbt_update_lastrec,
> .get_minrecs = xfs_allocbt_get_minrecs,
> .get_maxrecs = xfs_allocbt_get_maxrecs,
> .init_key_from_rec = xfs_allocbt_init_key_from_rec,
> @@ -511,7 +449,6 @@ const struct xfs_btree_ops xfs_bnobt_ops = {
> const struct xfs_btree_ops xfs_cntbt_ops = {
> .name = "cnt",
> .type = XFS_BTREE_TYPE_AG,
> - .geom_flags = XFS_BTGEO_LASTREC_UPDATE,
>
> .rec_len = sizeof(xfs_alloc_rec_t),
> .key_len = sizeof(xfs_alloc_key_t),
> @@ -525,7 +462,6 @@ const struct xfs_btree_ops xfs_cntbt_ops = {
> .set_root = xfs_allocbt_set_root,
> .alloc_block = xfs_allocbt_alloc_block,
> .free_block = xfs_allocbt_free_block,
> - .update_lastrec = xfs_allocbt_update_lastrec,
> .get_minrecs = xfs_allocbt_get_minrecs,
> .get_maxrecs = xfs_allocbt_get_maxrecs,
> .init_key_from_rec = xfs_allocbt_init_key_from_rec,
> diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> index d29547572a68..a5c4af148853 100644
> --- a/fs/xfs/libxfs/xfs_btree.c
> +++ b/fs/xfs/libxfs/xfs_btree.c
> @@ -1331,30 +1331,6 @@ xfs_btree_init_block_cur(
> xfs_btree_owner(cur));
> }
>
> -/*
> - * Return true if ptr is the last record in the btree and
> - * we need to track updates to this record. The decision
> - * will be further refined in the update_lastrec method.
> - */
> -STATIC int
> -xfs_btree_is_lastrec(
> - struct xfs_btree_cur *cur,
> - struct xfs_btree_block *block,
> - int level)
> -{
> - union xfs_btree_ptr ptr;
> -
> - if (level > 0)
> - return 0;
> - if (!(cur->bc_ops->geom_flags & XFS_BTGEO_LASTREC_UPDATE))
> - return 0;
> -
> - xfs_btree_get_sibling(cur, block, &ptr, XFS_BB_RIGHTSIB);
> - if (!xfs_btree_ptr_is_null(cur, &ptr))
> - return 0;
> - return 1;
> -}
> -
> STATIC void
> xfs_btree_buf_to_ptr(
> struct xfs_btree_cur *cur,
> @@ -2420,15 +2396,6 @@ xfs_btree_update(
> xfs_btree_copy_recs(cur, rp, rec, 1);
> xfs_btree_log_recs(cur, bp, ptr, ptr);
>
> - /*
> - * If we are tracking the last record in the tree and
> - * we are at the far right edge of the tree, update it.
> - */
> - if (xfs_btree_is_lastrec(cur, block, 0)) {
> - cur->bc_ops->update_lastrec(cur, block, rec,
> - ptr, LASTREC_UPDATE);
> - }
> -
> /* Pass new key value up to our parent. */
> if (xfs_btree_needs_key_update(cur, ptr)) {
> error = xfs_btree_update_keys(cur, 0);
> @@ -3617,15 +3584,6 @@ xfs_btree_insrec(
> goto error0;
> }
>
> - /*
> - * If we are tracking the last record in the tree and
> - * we are at the far right edge of the tree, update it.
> - */
> - if (xfs_btree_is_lastrec(cur, block, level)) {
> - cur->bc_ops->update_lastrec(cur, block, rec,
> - ptr, LASTREC_INSREC);
> - }
> -
> /*
> * Return the new block number, if any.
> * If there is one, give back a record value and a cursor too.
> @@ -3983,15 +3941,6 @@ xfs_btree_delrec(
> xfs_btree_set_numrecs(block, --numrecs);
> xfs_btree_log_block(cur, bp, XFS_BB_NUMRECS);
>
> - /*
> - * If we are tracking the last record in the tree and
> - * we are at the far right edge of the tree, update it.
> - */
> - if (xfs_btree_is_lastrec(cur, block, level)) {
> - cur->bc_ops->update_lastrec(cur, block, NULL,
> - ptr, LASTREC_DELREC);
> - }
> -
> /*
> * We're at the root level. First, shrink the root block in-memory.
> * Try to get rid of the next level down. If we can't then there's
> diff --git a/fs/xfs/libxfs/xfs_btree.h b/fs/xfs/libxfs/xfs_btree.h
> index f93374278aa1..10b7ddc3b2b3 100644
> --- a/fs/xfs/libxfs/xfs_btree.h
> +++ b/fs/xfs/libxfs/xfs_btree.h
> @@ -154,12 +154,6 @@ struct xfs_btree_ops {
> int *stat);
> int (*free_block)(struct xfs_btree_cur *cur, struct xfs_buf *bp);
>
> - /* update last record information */
> - void (*update_lastrec)(struct xfs_btree_cur *cur,
> - const struct xfs_btree_block *block,
> - const union xfs_btree_rec *rec,
> - int ptr, int reason);
> -
> /* records in block/level */
> int (*get_minrecs)(struct xfs_btree_cur *cur, int level);
> int (*get_maxrecs)(struct xfs_btree_cur *cur, int level);
> @@ -222,15 +216,7 @@ struct xfs_btree_ops {
> };
>
> /* btree geometry flags */
> -#define XFS_BTGEO_LASTREC_UPDATE (1U << 0) /* track last rec externally */
> -#define XFS_BTGEO_OVERLAPPING (1U << 1) /* overlapping intervals */
> -
> -/*
> - * Reasons for the update_lastrec method to be called.
> - */
> -#define LASTREC_UPDATE 0
> -#define LASTREC_INSREC 1
> -#define LASTREC_DELREC 2
> +#define XFS_BTGEO_OVERLAPPING (1U << 0) /* overlapping intervals */
>
>
> union xfs_btree_irec {
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