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Date: Fri, 25 Jul 2014 16:25:46 +0200
From: Michal Hocko <mhocko@...e.cz>
To: Hugh Dickins <hughd@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Sasha Levin <sasha.levin@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>,
Konstantin Khlebnikov <koct9i@...il.com>,
Johannes Weiner <hannes@...xchg.org>,
Michel Lespinasse <walken@...gle.com>,
Lukas Czerner <lczerner@...hat.com>,
Dave Jones <davej@...hat.com>, linux-mm@...ck.org,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] shmem: fix faulting into a hole, not taking i_mutex
On Tue 15-07-14 03:31:11, Hugh Dickins wrote:
> f00cdc6df7d7 ("shmem: fix faulting into a hole while it's punched") was
> buggy: Sasha sent a lockdep report to remind us that grabbing i_mutex in
> the fault path is a no-no (write syscall may already hold i_mutex while
> faulting user buffer).
>
> We tried a completely different approach (see following patch) but that
> proved inadequate: good enough for a rational workload, but not good
> enough against trinity - which forks off so many mappings of the object
> that contention on i_mmap_mutex while hole-puncher holds i_mutex builds
> into serious starvation when concurrent faults force the puncher to fall
> back to single-page unmap_mapping_range() searches of the i_mmap tree.
>
> So return to the original umbrella approach, but keep away from i_mutex
> this time. We really don't want to bloat every shmem inode with a new
> mutex or completion, just to protect this unlikely case from trinity.
> So extend the original with wait_queue_head on stack at the hole-punch
> end, and wait_queue item on the stack at the fault end.
>
> This involves further use of i_lock to guard against the races: lockdep
> has been happy so far, and I see fs/inode.c:unlock_new_inode() holds
> i_lock around wake_up_bit(), which is comparable to what we do here.
> i_lock is more convenient, but we could switch to shmem's info->lock.
>
> This issue has been tagged with CVE-2014-4171, which will require
> f00cdc6df7d7 and this and the following patch to be backported: we
> suggest to 3.1+, though in fact the trinity forkbomb effect might go
> back as far as 2.6.16, when madvise(,,MADV_REMOVE) came in - or might
> not, since much has changed, with i_mmap_mutex a spinlock before 3.0.
> Anyone running trinity on 3.0 and earlier? I don't think we need care.
>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Reported-by: Sasha Levin <sasha.levin@...cle.com>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: Konstantin Khlebnikov <koct9i@...il.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Lukas Czerner <lczerner@...hat.com>
> Cc: Dave Jones <davej@...hat.com>
> Cc: <stable@...r.kernel.org> [3.1+]
Feel free to add
Reviewed-by: Michal Hocko <mhocko@...e.cz>
for the UNINTERUPTIBLE sleep version.
> ---
> Please replace mmotm's
> revert-shmem-fix-faulting-into-a-hole-while-its-punched.patch
> by this patch.
>
> mm/shmem.c | 78 ++++++++++++++++++++++++++++++++++-----------------
> 1 file changed, 52 insertions(+), 26 deletions(-)
>
> --- 3.16-rc5/mm/shmem.c 2014-07-06 13:25:19.688009119 -0700
> +++ 3.16-rc5+/mm/shmem.c 2014-07-14 20:34:28.196153828 -0700
> @@ -85,7 +85,7 @@ static struct vfsmount *shm_mnt;
> * a time): we would prefer not to enlarge the shmem inode just for that.
> */
> struct shmem_falloc {
> - int mode; /* FALLOC_FL mode currently operating */
> + wait_queue_head_t *waitq; /* faults into hole wait for punch to end */
> pgoff_t start; /* start of range currently being fallocated */
> pgoff_t next; /* the next page offset to be fallocated */
> pgoff_t nr_falloced; /* how many new pages have been fallocated */
> @@ -760,7 +760,7 @@ static int shmem_writepage(struct page *
> spin_lock(&inode->i_lock);
> shmem_falloc = inode->i_private;
> if (shmem_falloc &&
> - !shmem_falloc->mode &&
> + !shmem_falloc->waitq &&
> index >= shmem_falloc->start &&
> index < shmem_falloc->next)
> shmem_falloc->nr_unswapped++;
> @@ -1248,38 +1248,58 @@ static int shmem_fault(struct vm_area_st
> * Trinity finds that probing a hole which tmpfs is punching can
> * prevent the hole-punch from ever completing: which in turn
> * locks writers out with its hold on i_mutex. So refrain from
> - * faulting pages into the hole while it's being punched, and
> - * wait on i_mutex to be released if vmf->flags permits.
> + * faulting pages into the hole while it's being punched. Although
> + * shmem_undo_range() does remove the additions, it may be unable to
> + * keep up, as each new page needs its own unmap_mapping_range() call,
> + * and the i_mmap tree grows ever slower to scan if new vmas are added.
> + *
> + * It does not matter if we sometimes reach this check just before the
> + * hole-punch begins, so that one fault then races with the punch:
> + * we just need to make racing faults a rare case.
> + *
> + * The implementation below would be much simpler if we just used a
> + * standard mutex or completion: but we cannot take i_mutex in fault,
> + * and bloating every shmem inode for this unlikely case would be sad.
> */
> if (unlikely(inode->i_private)) {
> struct shmem_falloc *shmem_falloc;
>
> spin_lock(&inode->i_lock);
> shmem_falloc = inode->i_private;
> - if (!shmem_falloc ||
> - shmem_falloc->mode != FALLOC_FL_PUNCH_HOLE ||
> - vmf->pgoff < shmem_falloc->start ||
> - vmf->pgoff >= shmem_falloc->next)
> - shmem_falloc = NULL;
> - spin_unlock(&inode->i_lock);
> - /*
> - * i_lock has protected us from taking shmem_falloc seriously
> - * once return from shmem_fallocate() went back up that stack.
> - * i_lock does not serialize with i_mutex at all, but it does
> - * not matter if sometimes we wait unnecessarily, or sometimes
> - * miss out on waiting: we just need to make those cases rare.
> - */
> - if (shmem_falloc) {
> + if (shmem_falloc &&
> + shmem_falloc->waitq &&
> + vmf->pgoff >= shmem_falloc->start &&
> + vmf->pgoff < shmem_falloc->next) {
> + wait_queue_head_t *shmem_falloc_waitq;
> + DEFINE_WAIT(shmem_fault_wait);
> +
> + ret = VM_FAULT_NOPAGE;
> if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
> !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
> + /* It's polite to up mmap_sem if we can */
> up_read(&vma->vm_mm->mmap_sem);
> - mutex_lock(&inode->i_mutex);
> - mutex_unlock(&inode->i_mutex);
> - return VM_FAULT_RETRY;
> + ret = VM_FAULT_RETRY;
> }
> - /* cond_resched? Leave that to GUP or return to user */
> - return VM_FAULT_NOPAGE;
> +
> + shmem_falloc_waitq = shmem_falloc->waitq;
> + prepare_to_wait(shmem_falloc_waitq, &shmem_fault_wait,
> + TASK_KILLABLE);
> + spin_unlock(&inode->i_lock);
> + schedule();
> +
> + /*
> + * shmem_falloc_waitq points into the shmem_fallocate()
> + * stack of the hole-punching task: shmem_falloc_waitq
> + * is usually invalid by the time we reach here, but
> + * finish_wait() does not dereference it in that case;
> + * though i_lock needed lest racing with wake_up_all().
> + */
> + spin_lock(&inode->i_lock);
> + finish_wait(shmem_falloc_waitq, &shmem_fault_wait);
> + spin_unlock(&inode->i_lock);
> + return ret;
> }
> + spin_unlock(&inode->i_lock);
> }
>
> error = shmem_getpage(inode, vmf->pgoff, &vmf->page, SGP_CACHE, &ret);
> @@ -1774,13 +1794,13 @@ static long shmem_fallocate(struct file
>
> mutex_lock(&inode->i_mutex);
>
> - shmem_falloc.mode = mode & ~FALLOC_FL_KEEP_SIZE;
> -
> if (mode & FALLOC_FL_PUNCH_HOLE) {
> struct address_space *mapping = file->f_mapping;
> loff_t unmap_start = round_up(offset, PAGE_SIZE);
> loff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;
> + DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
>
> + shmem_falloc.waitq = &shmem_falloc_waitq;
> shmem_falloc.start = unmap_start >> PAGE_SHIFT;
> shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
> spin_lock(&inode->i_lock);
> @@ -1792,8 +1812,13 @@ static long shmem_fallocate(struct file
> 1 + unmap_end - unmap_start, 0);
> shmem_truncate_range(inode, offset, offset + len - 1);
> /* No need to unmap again: hole-punching leaves COWed pages */
> +
> + spin_lock(&inode->i_lock);
> + inode->i_private = NULL;
> + wake_up_all(&shmem_falloc_waitq);
> + spin_unlock(&inode->i_lock);
> error = 0;
> - goto undone;
> + goto out;
> }
>
> /* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
> @@ -1809,6 +1834,7 @@ static long shmem_fallocate(struct file
> goto out;
> }
>
> + shmem_falloc.waitq = NULL;
> shmem_falloc.start = start;
> shmem_falloc.next = start;
> shmem_falloc.nr_falloced = 0;
>
> --
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--
Michal Hocko
SUSE Labs
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