Re: Large folios and filemap_get_folios_contig()

[Qu Wenruo <wqu@suse.com>]

在 2025/4/4 11:20, Vishal Moola (Oracle) 写道:
> On Fri, Apr 04, 2025 at 07:46:59AM +1030, Qu Wenruo wrote:
>>
[...]
>>>> Then I looked into the caller of filemap_get_folios_contig() inside
>>>> mm/gup, and it indeed does the correct skip.
>>>
>>> ... that code looks wrong to me.
>>
>> It looks like it's xas_find() is doing the correct skip by calling
>> xas_next_offset() -> xas_move_index() to skip the next one.
>>
>> But the filemap_get_folios_contig() only calls xas_next() by increasing the
>> index, not really skip to the next folio.
>>
>> Although I can be totally wrong as I'm not familiar with the xarray
>> internals at all.
> 
> Thanks for bringing this to my attention, it looks like this is due to a
> mistake during my folio conversion for this function. The xas_next()
> call tries to go to the next index, but if that index is part of a
> multi-index entry things get awkward if we don't manually account for the
> index shift of a large folio (which I missed).
> 
> Can you please try out this attached patch and see if it gets rid of
> the duplicate problem?

My reproducer no longer hang and the trace events indeed shows only one 
folio returned.

Thanks for the quick fix,
Qu

> 
>> However I totally agree the duplicated behavior (and the extra handling of
>> duplicated entries) looks very wrong.
>>
>> Thanks,
>> Qu