On 2018/8/30 上午9:57, Misono Tomohiro wrote: > On 2018/08/28 14:21, Qu Wenruo wrote: >> On 2018/8/24 下午4:09, Misono Tomohiro wrote: >> [snip] >>>> >>>> BTW, what's the possibility of such problem in your test environment? >>> >>> It's like one in several times. >>> It may depend on hardware performance? (the machine is not so fast), >>> >>> I also noticed following warning happens too (not always): >>> >> >> After digging into the case, it's more complex than just my patch. >> >> Firstly, we lacks a lot of underflow check when modifying bytes_may_use. >> So we need to do all the underflow detection for every modifier of >> bytes_may_use. >> >> Secondly, btrfs_cross_ref_exist() check makes NODATACOW check in >> __btrfs_buffered_write() unreliable. >> >> For the following case, at __btrfs_buffered_write() time we're pretty >> sure we could do NODATACOW, but at sync time, due to cloned range, >> btrfs_cross_ref_exist() would detect reflinked prealloc extent, then >> falls back to CoW, and finally cause bytes_may_use underflow: >> >> --- >> mkfs.btrfs -f $dev > $full_log >> >> mount $dev $mnt -o nospace_cache >> btrfs quota enable $mnt >> btrfs quota rescan -w $mnt >> >> xfs_io -f -c "falloc 0 2M" $mnt/file1 > /dev/null >> xfs_io -c "pwrite -b 1M 0 1M" $mnt/file1 > /dev/null >> xfs_io -c "reflink $mnt/file1 1M 4M 1M" $mnt/file1 > /dev/null >> sync >> ---- > > Thanks for the explanation. I will try to understand the relevant code.
Feel free to ask if there is anything I forgot to explain. > >> >> Even without my patch, the "pwrite" command is still CoWed, which could >> be avoided. >> And that's the reason my patch is causing the underflow. >> >> To fix this, we need more accurate btrfs_cross_ref_exist() check, not >> only for @disk_bytenr but also check @len. >> >> Or we could try to flush the whole inode in clone_range() so we could go >> through NOCOW routine before clone really happens. > > So as your RFC patch does not work, the option is first one? Yes, and it may be more complex to do that, while even fixed we could only get diminishing return. Thanks, Qu > > Thanks, > Misono > >> >> Thanks, >> Qu >> >
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