On 2/23/22 15:33, Matthew Wilcox wrote:
> On Wed, Feb 23, 2022 at 02:54:43PM +0100, Vlastimil Babka wrote:
>> 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 :)
>
> [...]
>
>> 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.
>
> I figured it out!
>
> In v5.15 (indeed, everything before commit 6b24ca4a1a8d), an order-9
> page is stored in 512 consecutive slots. The XArray stores 64 entries
> per level. So what happens is we start looking at index 0 and we walk
> down to the bottom of the tree and find the THP at index 0.
>
> xas.xa_index = head->index + thp_nr_pages(head) - 1;
> xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK;
>
> So we've advanced xas.xa_index to 511, but advanced xas.xa_offset to 63.
> Then we call xas_next() which calls __xas_next(), which moves us along to
> array index 64 while we think we're looking at index 512.
>
> We could make __xas_next() more resistant to this kind of abuse (by
> extracting the correct offset in the parent node from xa_index), but
> as you say, we're looking for a small fix for LTS. I suggest this
> will probably do the right thing:
Great!
Just so others are aware: the final fix is here:
https://lore.kernel.org/all/20220223155918.927140-1-willy@xxxxxxxxxxxxx/
> +++ b/mm/filemap.c
> @@ -2354,8 +2354,7 @@ static void filemap_get_read_batch(struct address_space *mapping,
> break;
> if (PageReadahead(head))
> break;
> - xas.xa_index = head->index + thp_nr_pages(head) - 1;
> - xas.xa_offset = (xas.xa_index >> xas.xa_shift) & XA_CHUNK_MASK;
> + xas_set(&xas, head->index + thp_nr_pages(head) - 1);
> continue;
> put_page:
> put_page(head);
>
> but I'll start trying the reproducer now.
>
>
