From de21fd427379d404b10a6a4b4563fd38fbbb1db0 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Wed, 23 Jul 2014 14:00:10 -0700 Subject: shmem: fix faulting into a hole, not taking i_mutex commit 8e205f779d1443a94b5ae81aa359cb535dd3021e upstream. Commit 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 commit 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 Reported-by: Sasha Levin Tested-by: Sasha Levin Cc: Vlastimil Babka Cc: Konstantin Khlebnikov Cc: Johannes Weiner Cc: Lukas Czerner Cc: Dave Jones Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Greg Kroah-Hartman Signed-off-by: Ben Hutchings --- mm/shmem.c | 64 ++++++++++++++++++++++++++++++++++++++++++-------------------- 1 file changed, 44 insertions(+), 20 deletions(-) diff --git a/mm/shmem.c b/mm/shmem.c index 042b4afbd68..ec7244bb03f 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -82,6 +82,7 @@ static struct vfsmount *shm_mnt; * a time): we would prefer not to enlarge the shmem inode just for that. */ struct shmem_falloc { + 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 */ }; @@ -1074,37 +1075,57 @@ static int shmem_fault(struct vm_area_struct *vma, struct vm_fault *vmf) * 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_truncate_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 || - 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 vmtruncate_range() 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 && + 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_UNINTERRUPTIBLE); + spin_unlock(&inode->i_lock); + schedule(); + + /* + * shmem_falloc_waitq points into the vmtruncate_range() + * 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); @@ -1135,7 +1156,9 @@ int vmtruncate_range(struct inode *inode, loff_t lstart, loff_t lend) struct address_space *mapping = inode->i_mapping; loff_t unmap_start = round_up(lstart, PAGE_SIZE); loff_t unmap_end = round_down(1 + lend, 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); @@ -1150,6 +1173,7 @@ int vmtruncate_range(struct inode *inode, loff_t lstart, loff_t lend) spin_lock(&inode->i_lock); inode->i_private = NULL; + wake_up_all(&shmem_falloc_waitq); spin_unlock(&inode->i_lock); } mutex_unlock(&inode->i_mutex); -- cgit v1.2.3-18-g5258