On Fri, 26 Nov 2021, Lukas Czerner wrote: > > I've noticed unusual test failure in e2fsprogs testsuite > (m_assume_storage_prezeroed) where we use mke2fs to create a file system > on loop device backed in file on tmpfs. For some reason sometimes the > resulting file number of allocated blocks (stat -c '%b' /tmp/file) differs, > but it really should not. > > I was trying to create a simplified reproducer and noticed the following > behavior on mainline kernel (v5.16-rc2-54-g5d9f4cf36721) > > # truncate -s16M /tmp/file > # stat -c '%b' /tmp/file > 0 > > # losetup -f /tmp/file > # stat -c '%b' /tmp/file > 672 > > That alone is a little unexpected since the file is really supposed to > be empty and when copied out of the tmpfs, it really is empty. But the > following is even more weird. > > We have a loop setup from above, so let's assume it's /dev/loop0. The > following should be executed in quick succession, like in a script. > > # dd if=/dev/zero of=/dev/loop0 bs=4k > # blkdiscard -f /dev/loop0 > # stat -c '%b' /tmp/file > 0 > # sleep 1 > # stat -c '%b' /tmp/file > 672 > > Is that expected behavior ? From what I've seen when I use mkfs instead > of this simplified example the number of blocks allocated as reported by > stat can vary a quite a lot given more complex operations. The file itself > does not seem to be corrupted in any way, so it is likely just an > accounting problem. > > Any idea what is going on there ? I have half an answer; but maybe you worked it all out meanwhile anyway. Yes, it happens like that for me too: 672 (but 216 on an old installation). Half the answer is that funny code at the head of shmem_file_read_iter(): /* * Might this read be for a stacking filesystem? Then when reading * holes of a sparse file, we actually need to allocate those pages, * and even mark them dirty, so it cannot exceed the max_blocks limit. */ if (!iter_is_iovec(to)) sgp = SGP_CACHE; which allocates pages to the tmpfs for reads from /dev/loop0; whereas normally a read of a sparse tmpfs file would just give zeroes without allocating. [Do we still need that code? Mikulas asked 18 months ago, and I never responded (sorry) because I failed to arrive at an informed answer. It comes from a time while unionfs on tmpfs was actively developing, and solved a real problem then; but by the time it went into tmpfs, unionfs had already been persuaded to proceed differently, and no longer needed it. I kept it in for indeterminate other stacking FSs, but it's probably just culted cargo, doing more harm than good. I suspect the best thing to do is, after the 5.17 merge window closes, revive Mikulas's patch to delete it and see if anyone complains.] But what is asynchronously reading /dev/loop0 (instantiating pages initially, and reinstantiating them after blkdiscard)? I assume it's some block device tracker, trying to read capacity and/or partition table; whether from inside or outside the kernel, I expect you'll guess much better than I can. Hugh
