xfs: fix race while discarding buffers [V4]
While xfs_buftarg_shrink() is freeing buffers from the dispose list (filled with buffers from lru list), there is a possibility to have xfs_buf_stale() racing with it, and removing buffers from dispose list before xfs_buftarg_shrink() does it. This happens because xfs_buftarg_shrink() handle the dispose list without locking and the test condition in xfs_buf_stale() checks for the buffer being in *any* list: if (!list_empty(&bp->b_lru)) If the buffer happens to be on dispose list, this causes the buffer counter of lru list (btp->bt_lru_nr) to be decremented twice (once in xfs_buftarg_shrink() and another in xfs_buf_stale()) causing a wrong account usage of the lru list. This may cause xfs_buftarg_shrink() to return a wrong value to the memory shrinker shrink_slab(), and such account error may also cause an underflowed value to be returned; since the counter is lower than the current number of items in the lru list, a decrement may happen when the counter is 0, causing an underflow on the counter. The fix uses a new flag field (and a new buffer flag) to serialize buffer handling during the shrink process. The new flag field has been designed to use btp->bt_lru_lock/unlock instead of xfs_buf_lock/unlock mechanism. dchinner, sandeen, aquini and aris also deserve credits for this. Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com> Reviewed-by: Ben Myers <bpm@sgi.com> Reviewed-by: Dave Chinner <dchinner@redhat.com> Signed-off-by: Ben Myers <bpm@sgi.com>
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@ -96,6 +96,7 @@ xfs_buf_lru_add(
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atomic_inc(&bp->b_hold);
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list_add_tail(&bp->b_lru, &btp->bt_lru);
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btp->bt_lru_nr++;
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bp->b_lru_flags &= ~_XBF_LRU_DISPOSE;
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}
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spin_unlock(&btp->bt_lru_lock);
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}
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@ -154,7 +155,8 @@ xfs_buf_stale(
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struct xfs_buftarg *btp = bp->b_target;
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spin_lock(&btp->bt_lru_lock);
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if (!list_empty(&bp->b_lru)) {
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if (!list_empty(&bp->b_lru) &&
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!(bp->b_lru_flags & _XBF_LRU_DISPOSE)) {
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list_del_init(&bp->b_lru);
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btp->bt_lru_nr--;
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atomic_dec(&bp->b_hold);
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@ -1501,6 +1503,7 @@ xfs_buftarg_shrink(
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*/
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list_move(&bp->b_lru, &dispose);
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btp->bt_lru_nr--;
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bp->b_lru_flags |= _XBF_LRU_DISPOSE;
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}
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spin_unlock(&btp->bt_lru_lock);
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@ -38,27 +38,28 @@ typedef enum {
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XBRW_ZERO = 3, /* Zero target memory */
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} xfs_buf_rw_t;
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#define XBF_READ (1 << 0) /* buffer intended for reading from device */
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#define XBF_WRITE (1 << 1) /* buffer intended for writing to device */
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#define XBF_READ_AHEAD (1 << 2) /* asynchronous read-ahead */
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#define XBF_ASYNC (1 << 4) /* initiator will not wait for completion */
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#define XBF_DONE (1 << 5) /* all pages in the buffer uptodate */
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#define XBF_STALE (1 << 6) /* buffer has been staled, do not find it */
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#define XBF_READ (1 << 0) /* buffer intended for reading from device */
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#define XBF_WRITE (1 << 1) /* buffer intended for writing to device */
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#define XBF_READ_AHEAD (1 << 2) /* asynchronous read-ahead */
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#define XBF_ASYNC (1 << 4) /* initiator will not wait for completion */
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#define XBF_DONE (1 << 5) /* all pages in the buffer uptodate */
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#define XBF_STALE (1 << 6) /* buffer has been staled, do not find it */
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/* I/O hints for the BIO layer */
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#define XBF_SYNCIO (1 << 10)/* treat this buffer as synchronous I/O */
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#define XBF_FUA (1 << 11)/* force cache write through mode */
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#define XBF_FLUSH (1 << 12)/* flush the disk cache before a write */
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#define XBF_SYNCIO (1 << 10)/* treat this buffer as synchronous I/O */
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#define XBF_FUA (1 << 11)/* force cache write through mode */
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#define XBF_FLUSH (1 << 12)/* flush the disk cache before a write */
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/* flags used only as arguments to access routines */
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#define XBF_TRYLOCK (1 << 16)/* lock requested, but do not wait */
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#define XBF_UNMAPPED (1 << 17)/* do not map the buffer */
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#define XBF_TRYLOCK (1 << 16)/* lock requested, but do not wait */
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#define XBF_UNMAPPED (1 << 17)/* do not map the buffer */
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/* flags used only internally */
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#define _XBF_PAGES (1 << 20)/* backed by refcounted pages */
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#define _XBF_KMEM (1 << 21)/* backed by heap memory */
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#define _XBF_DELWRI_Q (1 << 22)/* buffer on a delwri queue */
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#define _XBF_COMPOUND (1 << 23)/* compound buffer */
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#define _XBF_PAGES (1 << 20)/* backed by refcounted pages */
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#define _XBF_KMEM (1 << 21)/* backed by heap memory */
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#define _XBF_DELWRI_Q (1 << 22)/* buffer on a delwri queue */
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#define _XBF_COMPOUND (1 << 23)/* compound buffer */
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#define _XBF_LRU_DISPOSE (1 << 24)/* buffer being discarded */
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typedef unsigned int xfs_buf_flags_t;
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@ -72,12 +73,13 @@ typedef unsigned int xfs_buf_flags_t;
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{ XBF_SYNCIO, "SYNCIO" }, \
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{ XBF_FUA, "FUA" }, \
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{ XBF_FLUSH, "FLUSH" }, \
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{ XBF_TRYLOCK, "TRYLOCK" }, /* should never be set */\
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{ XBF_TRYLOCK, "TRYLOCK" }, /* should never be set */\
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{ XBF_UNMAPPED, "UNMAPPED" }, /* ditto */\
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{ _XBF_PAGES, "PAGES" }, \
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{ _XBF_KMEM, "KMEM" }, \
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{ _XBF_DELWRI_Q, "DELWRI_Q" }, \
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{ _XBF_COMPOUND, "COMPOUND" }
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{ _XBF_COMPOUND, "COMPOUND" }, \
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{ _XBF_LRU_DISPOSE, "LRU_DISPOSE" }
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typedef struct xfs_buftarg {
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dev_t bt_dev;
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@ -124,7 +126,12 @@ typedef struct xfs_buf {
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xfs_buf_flags_t b_flags; /* status flags */
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struct semaphore b_sema; /* semaphore for lockables */
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/*
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* concurrent access to b_lru and b_lru_flags are protected by
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* bt_lru_lock and not by b_sema
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*/
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struct list_head b_lru; /* lru list */
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xfs_buf_flags_t b_lru_flags; /* internal lru status flags */
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wait_queue_head_t b_waiters; /* unpin waiters */
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struct list_head b_list;
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struct xfs_perag *b_pag; /* contains rbtree root */
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