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Date: Wed, 29 Mar 2023 15:23:19 +0800 From: Baokun Li <libaokun1@...wei.com> To: Jan Kara <jack@...e.cz> CC: <linux-ext4@...r.kernel.org>, <tytso@....edu>, <adilger.kernel@...ger.ca>, <ritesh.list@...il.com>, <linux-kernel@...r.kernel.org>, <yi.zhang@...wei.com>, <yangerkun@...wei.com>, <yukuai3@...wei.com>, Baokun Li <libaokun1@...wei.com> Subject: Re: [PATCH] ext4: only update i_reserved_data_blocks on successful block allocation On 2023/3/28 18:00, Jan Kara wrote: > On Mon 27-03-23 21:09:42, Baokun Li wrote: >> On 2023/3/27 20:47, Jan Kara wrote: >>> On Sat 25-03-23 14:34:43, Baokun Li wrote: >>>> In our fault injection test, we create an ext4 file, migrate it to >>>> non-extent based file, then punch a hole and finally trigger a WARN_ON >>>> in the ext4_da_update_reserve_space(): >>>> >>>> EXT4-fs warning (device sda): ext4_da_update_reserve_space:369: >>>> ino 14, used 11 with only 10 reserved data blocks >>>> >>>> When writing back a non-extent based file, if we enable delalloc, the >>>> number of reserved blocks will be subtracted from the number of blocks >>>> mapped by ext4_ind_map_blocks(), and the extent status tree will be >>>> updated. We update the extent status tree by first removing the old >>>> extent_status and then inserting the new extent_status. If the block range >>>> we remove happens to be in an extent, then we need to allocate another >>>> extent_status with ext4_es_alloc_extent(). >>>> >>>> use old to remove to add new >>>> |----------|------------|------------| >>>> old extent_status >>>> >>>> The problem is that the allocation of a new extent_status failed due to a >>>> fault injection, and __es_shrink() did not get free memory, resulting in >>>> a return of -ENOMEM. Then do_writepages() retries after receiving -ENOMEM, >>>> we map to the same extent again, and the number of reserved blocks is again >>>> subtracted from the number of blocks in that extent. Since the blocks in >>>> the same extent are subtracted twice, we end up triggering WARN_ON at >>>> ext4_da_update_reserve_space() because used > ei->i_reserved_data_blocks. >>> Hum, but this second call to ext4_map_blocks() should find already allocated >>> blocks in the indirect block and thus should not be subtracting >>> ei->i_reserved_data_blocks for the second time. What am I missing? >>> >>> Honza >>> >> ext4_map_blocks >> 1. Lookup extent status tree firstly >> goto found; >> 2. get the block without requesting a new file system block. >> found: >> 3. ceate and map the block >> >> When we call ext4_map_blocks() for the second time, we directly find the >> corresponding blocks in the extent status tree, and then go directly to step >> 3, >> because our flag is brand new and therefore does not contain EXT4_MAP_MAPPED >> but contains EXT4_GET_BLOCKS_CREATE, thus subtracting >> ei->i_reserved_data_blocks >> for the second time. > Ah, I see. Thanks for explanation. But then the problem is deeper than just > a mismatch in number of reserved delalloc block. The problem really is that > if extent status tree update fails, we have inconsistency between what is > stored in the extent status tree and what is stored on disk. And that can > cause even data corruption issues in some cases. The scenario we encountered was this: ``` write: ext4_es_insert_delayed_block [0/16) 576460752303423487 (U,D) writepages: alloc lblk 11 pblk 35328 [0/16) 576460752303423487 (U,D) -- remove block 11 from extent [0/11) 576460752303423487 (U,D,R) + (Newly allocated)[12/4) 549196775151 (U,D,R) --Failure to allocate memory for a new extent will undo as: [0/16) 576460752303423487 (U,D,R) -- if success insert block 11 to extent status tree [0/11) 576460752303423487 (U,D,R) + (Newly allocated)[11/1) 35328 (W) + [12/4) 549196775151 (U,D,R) U: UNWRITTEN D: DELAYED W: WRITTEN R: REFERENCED ``` When we fail to allocate a new extent, we don't map buffer and we don't do io_submit, so why is the extent tree in memory inconsistent with the one stored on disk? Am I missing something? I would appreciate it if you could explain under what cases and what kind of data corruption issues can be caused. > > So I think we rather need to work on handling of errors in extent status > tree operations. In the extent status tree, we have extents which we can > just drop without issues and extents we must not drop - this depends on the > extent's status - currently ext4_es_is_delayed() extents must stay, others > may be dropped but I'd wrap the decision in a helper function. > > I'm currently inclined towards the following: > > 1) Removal must never fail. If we need to split extent, we use GFP_NOFAIL > if we cannot just drop the second part of the split extent in case of > allocation failure. > > 2) Similarly if inserting extent that cannot be dropped, we use GFP_NOFAIL. > > 3) We do not try to "undo" failed operations like we currently do - with > the above rules we never loose information that cannot be restored. Totally agree! This solution looks very effective and clear, I will try to implement it. Thank you very much for your suggestion! > > And this should also fix the problem you've hit because in case of > allocation failure we may just end up with removed extent from the extent > status tree and thus we refetch info from the disk and find out blocks are > already allocated. > > Honza Reloading extent tree from disk I don't quite understand here, how do we handle reserved blocks? could you explain it in more detail? Logically, I think it is still necessary to update i_reserved_data_blocks only after a successful allocation. This is also done in ext4_ext_map_blocks(). Thanks again! -- With Best Regards, Baokun Li .
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