On 9.03.2018 11:01, robbieko wrote: > Nikolay Borisov 於 2018-03-07 19:15 寫到: >> On 7.03.2018 12:27, robbieko wrote: >>> Nikolay Borisov 於 2018-03-07 18:19 寫到: >>>> On 7.03.2018 10:20, robbieko wrote: >>>>> From: Robbie Ko <robbi...@synology.com> >>>>> >>>>> # mount /dev/vdb5 /mnt/btrfs >>>>> # dd if=/dev/zero bs=16K count=4 oflag=dsync of=/mnt/btrfs/file >>>>> # xfs_io -c "fiemap -v" /mnt/btrfs/file >>>>> /mnt/btrfs/file: >>>>> EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS >>>>> 0: [0..127]: 25088..25215 128 0x1 >>>>> >>>>> Run fiemap with fm_extent_count set to 0, we'll get wrong value 4 >>>>> instead of 1. >>>> >>>> Wrong value 4 instead of 1 for which exact column, the flags? State >>>> this >>>> explicitly. >>>> >>>> Also this seems a bit bogus since fiemap's documentation states: >>>> >>>> If fm_extent_count is zero, then the fm_extents[] array is ignored (no >>>> extents will be returned), and the fm_mapped_extents count will hold >>>> the >>>> number of extents needed in fm_extents[] to hold the file's current >>>> mapping. >>>> >>>> So when fm_extent_count we shouldn't really be returning anything from >>>> kernel. >>>> >>> >>> Sorry I did not explain clearly. >>> The value is fm_mapped_extents. >> >> But fm_mapped_extents is tagged as an OUT member, meaning the user has >> no job writing to it. >> > > > [BUG] > fm_mapped_extents is not correct when fm_extent_count is 0 > Like: > # mount /dev/vdb5 /mnt/btrfs > # dd if=/dev/zero bs=16K count=4 oflag=dsync of=/mnt/btrfs/file > # xfs_io -c "fiemap -v" /mnt/btrfs/file > /mnt/btrfs/file: > EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS > 0: [0..127]: 25088..25215 128 0x1 > > When user space wants to get the number of file extents, > set fm_extent_count to 0 to run fiemap and then read fm_mapped_extents. > > In the above example, fiemap will return with fm_mapped_extents set to 4 , > but it should be 1 since there's only one entry in the output.
This is slightly better. > > [REASON] > When fm_extent_count is 0, disko is not initialized correctly, > The value is 0 in this case, not the right bytenr. The value of what, be more explicit. " Also the problem seems to be that disko is only set if fieinfo->fi_extents_max is set. And this member is initialized, in the generic ioctl_fiemap function, to the value of used-passed fm_extent_count. So when the user passes 0 then fi_extent_max is also set to zero and this causes btrfs to not initialize disko at all. Eventually this leads emit_fiemap_extent being called with a bogus 'phys' argument preventing proper fiemap entries merging. " You see how complicated in fact the issue is, and your 2 line explanation is not really making it clear. > It will cause the fiemap merge mechanism to fail. > > [FIX] > Use correct disko. This is as uninformative as it gets... What about something like : "Move the disko initialization earlier in extent_fiemap making it independent of user-passed arguments, allowing emit_fiemap_extent to properly handle consecutive extent entries." > > Thanks. > Robbie Ko > >>> If fm_extent_count is zero, the fm_mapped_extents count will hold the >>> number of extents needed. >>> >>> >>>> >>>>> >>>>> [REASON] >>>>> When fm_extent_count is 0, disko is not initialized correctly, >>>>> The value is 0 in this case, not the right bytenr. >>>> >>>> This is too sparse, be more explicit i.e. that disko=0 is passed to >>>> emit_fiemap_extent which then leads to issues. >>>> >>>>> >>>>> [FIX] >>>>> Use correct disko. >>>>> >>>>> Signed-off-by: Robbie Ko <robbi...@synology.com> >>>>> --- >>>>> fs/btrfs/extent_io.c | 4 +--- >>>>> 1 file changed, 1 insertion(+), 3 deletions(-) >>>>> >>>>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c >>>>> index 012d638..066b6df 100644 >>>>> --- a/fs/btrfs/extent_io.c >>>>> +++ b/fs/btrfs/extent_io.c >>>>> @@ -4567,7 +4567,7 @@ int extent_fiemap(struct inode *inode, struct >>>>> fiemap_extent_info *fieinfo, >>>>> offset_in_extent = em_start - em->start; >>>>> em_end = extent_map_end(em); >>>>> em_len = em_end - em_start; >>>>> - disko = 0; >>>>> + disko = em->block_start + offset_in_extent; >>>>> flags = 0; >>>>> >>>>> /* >>>>> @@ -4590,8 +4590,6 @@ int extent_fiemap(struct inode *inode, struct >>>>> fiemap_extent_info *fieinfo, >>>>> u64 bytenr = em->block_start - >>>>> (em->start - em->orig_start); >>>>> >>>>> - disko = em->block_start + offset_in_extent; >>>>> - >>>>> /* >>>>> * As btrfs supports shared space, this information >>>>> * can be exported to userspace tools via >>>>> -- >>>>> 1.9.1 >>>>> >>>>> -- >>>>> To unsubscribe from this list: send the line "unsubscribe >>>>> linux-btrfs" in >>>>> the body of a message to majord...@vger.kernel.org >>>>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>>>> >>> >>> -- >>> To unsubscribe from this list: send the line "unsubscribe >>> linux-btrfs" in >>> the body of a message to majord...@vger.kernel.org >>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>> > > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
