[RFC -v2 5/8] ext4: Do not fail journal due to block allocator

From: Michal Hocko
Date: Tue Aug 18 2015 - 06:39:12 EST

From: Michal Hocko <mhocko@xxxxxxxx>

Since "mm: page_alloc: do not lock up GFP_NOFS allocations upon OOM"
memory allocator doesn't endlessly loop to satisfy low-order allocations
and instead fails them to allow callers to handle them gracefully.

Some of the callers are not yet prepared for this behavior though. ext4
block allocator relies solely on GFP_NOFS allocation requests and
allocation failures lead to aborting yournal too easily:

[ 345.028333] oom-trash: page allocation failure: order:0, mode:0x50
[ 345.028336] CPU: 1 PID: 8334 Comm: oom-trash Tainted: G W 4.0.0-nofs3-00006-gdfe9931f5f68 #588
[ 345.028337] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150428_134905-gandalf 04/01/2014
[ 345.028339] 0000000000000000 ffff880005a17708 ffffffff81538a54 ffffffff8107a40f
[ 345.028341] 0000000000000050 ffff880005a17798 ffffffff810fe854 0000000180000000
[ 345.028342] 0000000000000046 0000000000000000 ffffffff81a52100 0000000000000246
[ 345.028343] Call Trace:
[ 345.028348] [<ffffffff81538a54>] dump_stack+0x4f/0x7b
[ 345.028370] [<ffffffff810fe854>] warn_alloc_failed+0x12a/0x13f
[ 345.028373] [<ffffffff81101bd2>] __alloc_pages_nodemask+0x7f3/0x8aa
[ 345.028375] [<ffffffff810f9933>] pagecache_get_page+0x12a/0x1c9
[ 345.028390] [<ffffffffa005bc64>] ext4_mb_load_buddy+0x220/0x367 [ext4]
[ 345.028414] [<ffffffffa006014f>] ext4_free_blocks+0x522/0xa4c [ext4]
[ 345.028425] [<ffffffffa0054e14>] ext4_ext_remove_space+0x833/0xf22 [ext4]
[ 345.028434] [<ffffffffa005677e>] ext4_ext_truncate+0x8c/0xb0 [ext4]
[ 345.028441] [<ffffffffa00342bf>] ext4_truncate+0x20b/0x38d [ext4]
[ 345.028462] [<ffffffffa003573c>] ext4_evict_inode+0x32b/0x4c1 [ext4]
[ 345.028464] [<ffffffff8116d04f>] evict+0xa0/0x148
[ 345.028466] [<ffffffff8116dca8>] iput+0x1a1/0x1f0
[ 345.028468] [<ffffffff811697b4>] __dentry_kill+0x136/0x1a6
[ 345.028470] [<ffffffff81169a3e>] dput+0x21a/0x243
[ 345.028472] [<ffffffff81157cda>] __fput+0x184/0x19b
[ 345.028473] [<ffffffff81157d29>] ____fput+0xe/0x10
[ 345.028475] [<ffffffff8105a05f>] task_work_run+0x8a/0xa1
[ 345.028477] [<ffffffff810452f0>] do_exit+0x3c6/0x8dc
[ 345.028482] [<ffffffff8104588a>] do_group_exit+0x4d/0xb2
[ 345.028483] [<ffffffff8104eeeb>] get_signal+0x5b1/0x5f5
[ 345.028488] [<ffffffff81002202>] do_signal+0x28/0x5d0
[...]
[ 345.028624] EXT4-fs error (device hdb1) in ext4_free_blocks:4879: Out of memory
[ 345.033097] Aborting journal on device hdb1-8.
[ 345.036339] EXT4-fs (hdb1): Remounting filesystem read-only
[ 345.036344] EXT4-fs error (device hdb1) in ext4_reserve_inode_write:4834: Journal has aborted
[ 345.036766] EXT4-fs error (device hdb1) in ext4_reserve_inode_write:4834: Journal has aborted
[ 345.038583] EXT4-fs error (device hdb1) in ext4_ext_remove_space:3048: Journal has aborted
[ 345.049115] EXT4-fs error (device hdb1) in ext4_ext_truncate:4669: Journal has aborted
[ 345.050434] EXT4-fs error (device hdb1) in ext4_reserve_inode_write:4834: Journal has aborted
[ 345.053064] EXT4-fs error (device hdb1) in ext4_truncate:3668: Journal has aborted
[ 345.053582] EXT4-fs error (device hdb1) in ext4_reserve_inode_write:4834: Journal has aborted
[ 345.053946] EXT4-fs error (device hdb1) in ext4_orphan_del:2686: Journal has aborted
[ 345.055367] EXT4-fs error (device hdb1) in ext4_reserve_inode_write:4834: Journal has aborted

The failure is really premature because GFP_NOFS allocation context is
very restricted - especially in the fs metadata heavy loads. Before we
go with a more sofisticated solution, let's simply imitate the previous
behavior of non-failing NOFS allocation and use __GFP_NOFAIL for the
buddy block allocator. I wasn't able to trigger the issue with this
patch anymore.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
fs/ext4/mballoc.c | 52 ++++++++++++++++++++++++----------------------------
1 file changed, 24 insertions(+), 28 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 5b1613a54307..0360ea32c30f 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -992,9 +992,8 @@ static int ext4_mb_get_buddy_page_lock(struct super_block *sb,
block = group * 2;
pnum = block / blocks_per_page;
poff = block % blocks_per_page;
- page = find_or_create_page(inode->i_mapping, pnum, GFP_NOFS);
- if (!page)
- return -ENOMEM;
+ page = find_or_create_page(inode->i_mapping, pnum,
+ GFP_NOFS|__GFP_NOFAIL);
BUG_ON(page->mapping != inode->i_mapping);
e4b->bd_bitmap_page = page;
e4b->bd_bitmap = page_address(page) + (poff * sb->s_blocksize);
@@ -1006,9 +1005,8 @@ static int ext4_mb_get_buddy_page_lock(struct super_block *sb,

block++;
pnum = block / blocks_per_page;
- page = find_or_create_page(inode->i_mapping, pnum, GFP_NOFS);
- if (!page)
- return -ENOMEM;
+ page = find_or_create_page(inode->i_mapping, pnum,
+ GFP_NOFS|__GFP_NOFAIL);
BUG_ON(page->mapping != inode->i_mapping);
e4b->bd_buddy_page = page;
return 0;
@@ -1158,20 +1156,19 @@ ext4_mb_load_buddy(struct super_block *sb, ext4_group_t group,
* wait for it to initialize.
*/
page_cache_release(page);
- page = find_or_create_page(inode->i_mapping, pnum, GFP_NOFS);
- if (page) {
- BUG_ON(page->mapping != inode->i_mapping);
- if (!PageUptodate(page)) {
- ret = ext4_mb_init_cache(page, NULL);
- if (ret) {
- unlock_page(page);
- goto err;
- }
- mb_cmp_bitmaps(e4b, page_address(page) +
- (poff * sb->s_blocksize));
+ page = find_or_create_page(inode->i_mapping, pnum,
+ GFP_NOFS|__GFP_NOFAIL);
+ BUG_ON(page->mapping != inode->i_mapping);
+ if (!PageUptodate(page)) {
+ ret = ext4_mb_init_cache(page, NULL);
+ if (ret) {
+ unlock_page(page);
+ goto err;
}
- unlock_page(page);
+ mb_cmp_bitmaps(e4b, page_address(page) +
+ (poff * sb->s_blocksize));
}
+ unlock_page(page);
}
if (page == NULL) {
ret = -ENOMEM;
@@ -1194,18 +1191,17 @@ ext4_mb_load_buddy(struct super_block *sb, ext4_group_t group,
if (page == NULL || !PageUptodate(page)) {
if (page)
page_cache_release(page);
- page = find_or_create_page(inode->i_mapping, pnum, GFP_NOFS);
- if (page) {
- BUG_ON(page->mapping != inode->i_mapping);
- if (!PageUptodate(page)) {
- ret = ext4_mb_init_cache(page, e4b->bd_bitmap);
- if (ret) {
- unlock_page(page);
- goto err;
- }
+ page = find_or_create_page(inode->i_mapping, pnum,
+ GFP_NOFS|__GFP_NOFAIL);
+ BUG_ON(page->mapping != inode->i_mapping);
+ if (!PageUptodate(page)) {
+ ret = ext4_mb_init_cache(page, e4b->bd_bitmap);
+ if (ret) {
+ unlock_page(page);
+ goto err;
}
- unlock_page(page);
}
+ unlock_page(page);
}
if (page == NULL) {
ret = -ENOMEM;
--
2.5.0

--
Michal Hocko
SUSE Labs
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