On Tue, Jan 11, 2022 at 08:28:02PM -0800, Hugh Dickins wrote: > 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.] I for one wouldn't mind if tmpfs no longer instantiated cache pages for a read from a hole -- it's a little strange, since most disk filesystems (well ok xfs and ext4, haven't checked the others) don't do that. Anyone who really wants a preallocated page should probably be using fallocate or something... --D > 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
