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Date: Thu, 30 Jan 2020 23:04:56 +0530 From: Ritesh Harjani <riteshh@...ux.ibm.com> To: "Darrick J. Wong" <darrick.wong@...cle.com> Cc: jack@...e.cz, tytso@....edu, adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, hch@...radead.org, cmaiolino@...hat.com Subject: Re: [RFCv2 0/4] ext4: bmap & fiemap conversion to use iomap On 1/30/20 9:30 PM, Darrick J. Wong wrote: > On Tue, Jan 28, 2020 at 03:48:24PM +0530, Ritesh Harjani wrote: >> Hello All, >> >> Background >> ========== >> There are RFCv2 patches to move ext4 bmap & fiemap calls to use iomap APIs. >> This reduces the users of ext4_get_block API and thus a step towards getting >> rid of buffer_heads from ext4. Also reduces a lot of code by making use of >> existing iomap_ops (except for xattr implementation). >> >> Testing (done on ext4 master branch) >> ======== >> 'xfstests -g auto' passes with default mkfs/mount configuration >> (v/s which also pass with vanilla kernel without this patch). Except >> generic/473 which also failes on XFS. This seems to be the test case issue >> since it expects the data in slightly different way as compared to what iomap >> returns. >> Point 2.a below describes more about this. >> >> Observations/Review required >> ============================ >> 1. bmap related old v/s new method differences:- >> a. In case if addr > INT_MAX, it issues a warning and >> returns 0 as the block no. While earlier it used to return the >> truncated value with no warning. > > Good... > >> b. block no. is only returned in case of iomap->type is IOMAP_MAPPED, >> but not when iomap->type is IOMAP_UNWRITTEN. While with previously >> we used to get block no. for both of above cases. > > Assuming the only remaining usecase of bmap is to tell old bootloaders > where to find vmlinuz blocks on disk, I don't see much reason to map > unwritten blocks -- there's no data there, and if your bootloader writes > to the filesystem(!) then you can't read whatever was written there > anyway. Yes, no objection there. Just wanted to get it reviewed. > > Uh, can we put this ioctl on the deprecation list, please? :) > >> 2. Fiemap related old v/s new method differences:- >> a. iomap_fiemap returns the disk extent information in exact >> correspondence with start of user requested logical offset till the >> length requested by user. While in previous implementation the >> returned information used to give the complete extent information if >> the range requested by user lies in between the extent mapping. > > This is a topic of much disagreement. The FIEMAP documentation says > that the call must return data for the requested range, but *may* return > a mapping that extends beyond the requested range. > > XFS (and now iomap) only return data for the requested range, whereas > ext4 has (had?) the behavior you describe. generic/473 was an attempt > to enforce the ext4 behavior across all filesystems, but I put it in my > dead list and never run it. > > So it's a behavioral change, but the new behavior isn't forbidden. Sure, thanks. > >> b. iomap_fiemap adds the FIEMAP_EXTENT_LAST flag also at the last >> fiemap_extent mapping range requested by the user via fm_length ( >> if that has a valid mapped extent on the disk). > > That sounds like a bug. _LAST is supposed to be set on the last extent > in the file, not the last record in the queried dataset. Thought so too, sure will spend some time to try fixing it. > >> But if the user >> requested for more fm_length which could not be mapped in the last >> fiemap_extent, then the flag is not set. > > Yes... if there were more extents to map than there was space in the map > array, then _LAST should remain unset to encourage userspace to come > back for the rest of the mappings. > > (Unless maybe I'm misunderstanding here...) > >> e.g. output for above differences 2.a & 2.b >> =========================================== >> create a file with below cmds. >> 1. fallocate -o 0 -l 8K testfile.txt >> 2. xfs_io -c "pwrite 8K 8K" testfile.txt >> 3. check extent mapping:- xfs_io -c "fiemap -v" testfile.txt >> 4. run this binary on with and without these patches:- ./a.out (test_fiemap_diff.c) [4] >> >> o/p of xfs_io -c "fiemap -v" >> ============================================ >> With this patch on patched kernel:- >> testfile.txt: >> EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS >> 0: [0..15]: 122802736..122802751 16 0x800 >> 1: [16..31]: 122687536..122687551 16 0x1 >> >> without patch on vanilla kernel (no difference):- >> testfile.txt: >> EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS >> 0: [0..15]: 332211376..332211391 16 0x800 >> 1: [16..31]: 332722392..332722407 16 0x1 >> >> >> o/p of a.out without patch:- >> ================ >> riteshh-> ./a.out >> logical: [ 0.. 15] phys: 332211376..332211391 flags: 0x800 tot: 16 >> (0) extent flag = 2048 >> >> o/p of a.out with patch (both point 2.a & 2.b could be seen) >> ======================= >> riteshh-> ./a.out >> logical: [ 0.. 7] phys: 122802736..122802743 flags: 0x801 tot: 8 >> (0) extent flag = 2049 >> >> FYI - In test_fiemap_diff.c test we had >> a. fm_extent_count = 1 >> b. fm_start = 0 >> c. fm_length = 4K >> Whereas when we change fm_extent_count = 32, then we don't see any difference. >> >> e.g. output for above difference listed in point 1.b >> ==================================================== >> >> o/p without patch (block no returned for unwritten block as well) >> =========Testing IOCTL FIBMAP========= >> File size = 16384, blkcnt = 4, blocksize = 4096 >> 0 41526422 >> 1 41526423 >> 2 41590299 >> 3 41590300 >> >> o/p with patch (0 returned for unwritten block) >> =========Testing IOCTL FIBMAP========= >> File size = 16384, blkcnt = 4, blocksize = 4096 >> 0 0 0 >> 1 0 0 >> 2 15335942 29953 >> 3 15335943 29953 >> >> >> Summary:- >> ======== >> Due to some of the observational differences to user, listed above, >> requesting to please help with a careful review in moving this to iomap. >> Digging into some older threads, it looks like these differences should be fine, >> since the same tools have been working fine with XFS (which uses iomap based >> implementation) [1] >> Also as Ted suggested in [3]: Fiemap & bmap spec could be made based on the ext4 >> implementation. But since all the tools also work with xfs which uses iomap >> based fiemap, so we should be good there. > > <nod> Thanks for the worked example output. :) Thanks for the review. :) ritesh > > --D > >> >> References of some previous discussions: >> ======================================= >> [1]: https://www.spinics.net/lists/linux-fsdevel/msg128370.html >> [2]: https://www.spinics.net/lists/linux-fsdevel/msg127675.html >> [3]: https://www.spinics.net/lists/linux-fsdevel/msg128368.html >> [4]: https://raw.githubusercontent.com/riteshharjani/LinuxStudy/master/tools/test_fiemap_diff.c >> [RFCv1]: https://www.spinics.net/lists/linux-ext4/msg67077.html >> >> >> Ritesh Harjani (4): >> ext4: Add IOMAP_F_MERGED for non-extent based mapping >> ext4: Optimize ext4_ext_precache for 0 depth >> ext4: Move ext4 bmap to use iomap infrastructure. >> ext4: Move ext4_fiemap to use iomap infrastructure >> >> fs/ext4/extents.c | 288 +++++++--------------------------------------- >> fs/ext4/inline.c | 41 ------- >> fs/ext4/inode.c | 6 +- >> 3 files changed, 49 insertions(+), 286 deletions(-) >> >> -- >> 2.21.0 >>
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