Re: [f2fs-dev] [PATCH 3/9] f2fs: rework write preallocations

[Chao Yu <chao@kernel.org>]

On 2021/7/16 22:39, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> f2fs_write_begin() assumes that all blocks were preallocated by
> default unless FI_NO_PREALLOC is explicitly set.  This invites data
> corruption, as there are cases in which not all blocks are preallocated.
> Commit 47501f87c61a ("f2fs: preallocate DIO blocks when forcing
> buffered_io") fixed one case, but there are others remaining.

Could you please explain which cases we missed to handle previously?
then I can check those related logic before and after the rework.

> -			/*
> -			 * If force_buffere_io() is true, we have to allocate
> -			 * blocks all the time, since f2fs_direct_IO will fall
> -			 * back to buffered IO.
> -			 */
> -			if (!f2fs_force_buffered_io(inode, iocb, from) &&
> -					f2fs_lfs_mode(F2FS_I_SB(inode)))

We should keep this OPU DIO logic, otherwise, in lfs mode, write dio
will always allocate two block addresses for each 4k append IO.

I jsut test based on codes of last f2fs dev-test branch.

rm /mnt/f2fs/dio
dd if=/dev/zero  of=/mnt/f2fs/dio bs=4k count=4 oflag=direct

           <...>-763176  [001] ...1 177258.793370: f2fs_map_blocks: dev = (259,1), ino = 6, file offset = 0, start blkaddr = 0xe1a2e, len = 0x1, flags = 48,seg_type = 1, may_create = 1, err = 0
            <...>-763176  [001] ...1 177258.793462: f2fs_map_blocks: dev = (259,1), ino = 6, file offset = 0, start blkaddr = 0xe1a2f, len = 0x1, flags = 16,seg_type = 1, may_create = 1, err = 0
               dd-763176  [001] ...1 177258.793575: f2fs_map_blocks: dev = (259,1), ino = 6, file offset = 1, start blkaddr = 0xe1a30, len = 0x1, flags = 48,seg_type = 1, may_create = 1, err = 0
               dd-763176  [001] ...1 177258.793599: f2fs_map_blocks: dev = (259,1), ino = 6, file offset = 1, start blkaddr = 0xe1a31, len = 0x1, flags = 16,seg_type = 1, may_create = 1, err = 0
               dd-763176  [001] ...1 177258.793735: f2fs_map_blocks: dev = (259,1), ino = 6, file offset = 2, start blkaddr = 0xe1a32, len = 0x1, flags = 48,seg_type = 1, may_create = 1, err = 0
               dd-763176  [001] ...1 177258.793769: f2fs_map_blocks: dev = (259,1), ino = 6, file offset = 2, start blkaddr = 0xe1a33, len = 0x1, flags = 16,seg_type = 1, may_create = 1, err = 0
               dd-763176  [001] ...1 177258.793859: f2fs_map_blocks: dev = (259,1), ino = 6, file offset = 3, start blkaddr = 0xe1a34, len = 0x1, flags = 48,seg_type = 1, may_create = 1, err = 0
               dd-763176  [001] ...1 177258.793885: f2fs_map_blocks: dev = (259,1), ino = 6, file offset = 3, start blkaddr = 0xe1a35, len = 0x1, flags = 16,seg_type = 1, may_create = 1, err = 0

Thanks,


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