On Sun, Aug 30, 2015 at 6:31 AM, Dave Chinner <david@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx > > --- > 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 _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
