Hi,

we have found a bug involving CONFIG_READ_ONLY_THP_FOR_FS=y, introduced in 5.12 by cbd59c48ae2b ("mm/filemap: use head pages in generic_file_buffered_read") and apparently fixed in 5.17-rc1 by 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache") The latter commit is part of folio rework so likely not stable material, so it would be nice to have a small fix for e.g. 5.15 LTS. Preferably from someone who understands xarray :)

The bug was found while building nodejs16, which involves running some tests, first with a non-stripped node16 binary, and then stripping it. This has a good chance of the stripped result becoming corrupted and not executable anymore due to dynamic loader failures. It turns out that while executed during tests, CONFIG_READ_ONLY_THP_FOR_FS=y allows khugepaged to collapse the large executable mappings. Then /usr/bin/strip uses read() to process the binary and triggers a bug introduced in cbd59c48ae2b where if a read spans two or more collapsed THPs in the page cache, the first one will be read multiple times instead.

Takashi Iwai has bisected using the nodejs build scenario to commit cbd59c48ae2b.

I have distilled the scenario to the attached reproducer. There are some assumptions for it to work:

- the passed path for file it creates/works with should be on a filesystem such as xfs or ext4 that uses generic_file_buffered_read() - the kernel should have e6be37b2e7bd ("mm/huge_memory.c: add missing read-only THP checking in transparent_hugepage_enabled()") otherwise khugepaged will not recognize the reproducer's mm as thp eligible (it had to be some other mapping in nodejs that made it still possible to trigger this during bisect) - there's a pause to allow khugepaged to do its job, you can increase the speed as instructed and verify with /proc/pid/smaps and meminfo that the collapse in page cache has happened - if the bug is reproduced, the reproducer will complain like this: mismatch at offset 2097152: read value expected for offset 0 instead of 2097152

I've hacked some printk on top 5.16 (attached debug.patch) which gives this output:

i=0 page=ffffea0004340000 page_offset=0 uoff=0 bytes=2097152 i=1 page=ffffea0004340000 page_offset=0 uoff=0 bytes=2097152 i=2 page=ffffea0004340000 page_offset=0 uoff=0 bytes=0 i=3 page=ffffea0004340000 page_offset=0 uoff=0 bytes=0 i=4 page=ffffea0004340000 page_offset=0 uoff=0 bytes=0 i=5 page=ffffea0004340000 page_offset=0 uoff=0 bytes=0 i=6 page=ffffea0004340000 page_offset=0 uoff=0 bytes=0 i=7 page=ffffea0004340000 page_offset=0 uoff=0 bytes=0 i=8 page=ffffea0004470000 page_offset=2097152 uoff=0 bytes=0 i=9 page=ffffea0004470000 page_offset=2097152 uoff=0 bytes=0 i=10 page=ffffea0004470000 page_offset=2097152 uoff=0 bytes=0 i=11 page=ffffea0004470000 page_offset=2097152 uoff=0 bytes=0 i=12 page=ffffea0004470000 page_offset=2097152 uoff=0 bytes=0 i=13 page=ffffea0004470000 page_offset=2097152 uoff=0 bytes=0 i=14 page=ffffea0004470000 page_offset=2097152 uoff=0 bytes=0

It seems filemap_get_read_batch() should be returning pages ffffea0004340000 and ffffea0004470000 consecutively in the pvec, but returns the first one 8 times, so it's read twice and then the rest is just skipped over as it's beyond the requested read size.

I suspect these lines: xas.xa_index = head->index + thp_nr_pages(head) - 1; xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK;

commit 6b24ca4a1a8d changes those to xas_advance() (introduced one patch earlier), so some self-contained fix should be possible for prior kernels? But I don't understand xarray well enough.

My colleagues should be credited for initial analysis of the nodejs build scenario:

Analyzed-by: Adam Majer amajer@suse.com Analyzed-by: Dirk Mueller dmueller@suse.com Bisected-by: Takashi Iwai tiwai@suse.de

Thanks, Vlastimil