Message ID | 49BAD6D9.3010505@redhat.com |
---|---|
State | Superseded, archived |
Headers | show |
Eric Sandeen wrote: > This is for Red Hat bug 490026, > EXT4 panic, list corruption in ext4_mb_new_inode_pa > > (this was on backported ext4 from 2.6.29) > > We hit a BUG() in __list_add from ext4_mb_new_inode_pa() > because the list head pointed to a removed item: > > list_add corruption. next->prev should be ffff81042f2fe158, > but was 0000000000200200 > > (0000000000200200 is LIST_POISON2, set when the item is deleted) > > ext4_lock_group(sb, group) is supposed to protect this list for > each group, and a common code flow is this: > > ext4_get_group_no_and_offset(sb, pa->pa_pstart, &grp, NULL); > ext4_lock_group(sb, grp); > list_del(&pa->pa_group_list); > ext4_unlock_group(sb, grp); > > so its critical that we get the right group number back for > this pa->pa_pstart block. > > however, ext4_mb_put_pa passes in (pa->pa_pstart - 1) with a > comment, "-1 is to protect from crossing allocation group" > > Other list-manipulators do not use the "-1" so we have the > potential to lock the wrong group and race. Given how the > ext4_get_group_no_and_offset() function works, it doesn't seem > to me that the subtraction is correct. Hm, unless pa_pstart gets advanced to the point where it's in the next group when it's used up... might be more reading to do here. -Eric -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Eric Sandeen wrote: > Eric Sandeen wrote: >> This is for Red Hat bug 490026, >> EXT4 panic, list corruption in ext4_mb_new_inode_pa >> >> (this was on backported ext4 from 2.6.29) >> >> We hit a BUG() in __list_add from ext4_mb_new_inode_pa() >> because the list head pointed to a removed item: >> >> list_add corruption. next->prev should be ffff81042f2fe158, >> but was 0000000000200200 >> >> (0000000000200200 is LIST_POISON2, set when the item is deleted) >> >> ext4_lock_group(sb, group) is supposed to protect this list for >> each group, and a common code flow is this: >> >> ext4_get_group_no_and_offset(sb, pa->pa_pstart, &grp, NULL); >> ext4_lock_group(sb, grp); >> list_del(&pa->pa_group_list); >> ext4_unlock_group(sb, grp); >> >> so its critical that we get the right group number back for >> this pa->pa_pstart block. >> >> however, ext4_mb_put_pa passes in (pa->pa_pstart - 1) with a >> comment, "-1 is to protect from crossing allocation group" >> >> Other list-manipulators do not use the "-1" so we have the >> potential to lock the wrong group and race. Given how the >> ext4_get_group_no_and_offset() function works, it doesn't seem >> to me that the subtraction is correct. > > Hm, unless pa_pstart gets advanced to the point where it's in the next > group when it's used up... might be more reading to do here. Ok I think I was on the right track here. It looks like for group_pa (with pa_linear == 1), pa->pa_pstart is advanced as it's used (actually in ext4_mb_release_context(), but that's a detail) so by the time it is used up, pa->pa_pstart has advanced into the next group* and therefore subtracting one to find the group it belong(ed) to is correct. However, for inode_pa (with pa_linear == 0) only pa_free is decremented, and pa_pstart does not move. Therefore subtracting one from pa_pstart in ext4_mb_put_pa is actually grabbing the previous block group's lock in the inode case, and we open a race with other threads which are locking the correct group. I'll do a bit more testing/reading but I think that what we probably need is something like: static void ext4_mb_put_pa(struct ext4_allocation_context *ac, ...) { ... /* -1 is to protect from crossing allocation group */ if (pa->pa_linear) pa->pa_pstart--; ext4_get_group_no_and_offset(sb, pa->pa_pstart, &grp, NULL); ... Could probably come up with something clearer, but that's the gist of it. -Eric *i.e. if pa_start began at 0, and the group had 512 blocks, when all blocks are used, pa_start has advanced by 512, and "512" is the first block in the *next* group, so we need to trim one off in that case. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Index: linux-2.6/fs/ext4/mballoc.c =================================================================== --- linux-2.6.orig/fs/ext4/mballoc.c +++ linux-2.6/fs/ext4/mballoc.c @@ -3603,8 +3603,7 @@ static void ext4_mb_put_pa(struct ext4_a pa->pa_deleted = 1; spin_unlock(&pa->pa_lock); - /* -1 is to protect from crossing allocation group */ - ext4_get_group_no_and_offset(sb, pa->pa_pstart - 1, &grp, NULL); + ext4_get_group_no_and_offset(sb, pa->pa_pstart, &grp, NULL); /* * possible race:
This is for Red Hat bug 490026, EXT4 panic, list corruption in ext4_mb_new_inode_pa (this was on backported ext4 from 2.6.29) We hit a BUG() in __list_add from ext4_mb_new_inode_pa() because the list head pointed to a removed item: list_add corruption. next->prev should be ffff81042f2fe158, but was 0000000000200200 (0000000000200200 is LIST_POISON2, set when the item is deleted) ext4_lock_group(sb, group) is supposed to protect this list for each group, and a common code flow is this: ext4_get_group_no_and_offset(sb, pa->pa_pstart, &grp, NULL); ext4_lock_group(sb, grp); list_del(&pa->pa_group_list); ext4_unlock_group(sb, grp); so its critical that we get the right group number back for this pa->pa_pstart block. however, ext4_mb_put_pa passes in (pa->pa_pstart - 1) with a comment, "-1 is to protect from crossing allocation group" Other list-manipulators do not use the "-1" so we have the potential to lock the wrong group and race. Given how the ext4_get_group_no_and_offset() function works, it doesn't seem to me that the subtraction is correct. I've not been able to reproduce the bug, so this is by inspection. Signed-off-by: Eric Sandeen <sandeen@redhat.com> --- -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html