Re: [PATCH v6 18/18] arm64/mm: Automatically fold contpte mappings

[Ryan Roberts <ryan.roberts@arm.com>]

On 25/06/2024 14:06, Matthew Wilcox wrote:
> On Tue, Jun 25, 2024 at 01:41:02PM +0100, Ryan Roberts wrote:
>> On 25/06/2024 13:37, Baolin Wang wrote:
>>
>> [...]
>>
>>>>> For other filesystems, like ext4, I did not found the logic to determin what
>>>>> size of folio to allocate in writable mmap() path
>>>>
>>>> Yes I'd be keen to understand this to. When I was doing contpte, page cache
>>>> would only allocate large folios for readahead. So that's why I wouldn't have
>>>
>>> You mean non-large folios, right?
>>
>> No I mean that at the time I wrote contpte, the policy was to allocate an
>> order-0 folio for any writes that missed in the page cache, and allocate large
>> folios only when doing readahead from storage into page cache. The test that is
>> regressing is doing writes.
> 
> mmap() faults also use readahead.
> 
> filemap_fault():
> 
>         folio = filemap_get_folio(mapping, index);
>         if (likely(!IS_ERR(folio))) {
>                 if (!(vmf->flags & FAULT_FLAG_TRIED))
>                         fpin = do_async_mmap_readahead(vmf, folio);
> which does:
>         if (folio_test_readahead(folio)) {
>                 fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>                 page_cache_async_ra(&ractl, folio, ra->ra_pages);
> 
> which has been there in one form or another since 2007 (3ea89ee86a82).

OK sounds like I'm probably misremembering something I read on LWN... You're
saying that its been the case for a while that if we take a write fault for a
portion of a file, then we will still end up taking the readahead path and
allocating a large folio (filesystem permitting)? Does that apply in the case
where the file has never been touched but only ftruncate'd, as is happening in
this test? There is obviously no need for IO in that case, but have we always
taken a path where a large folio may be allocated for it? I thought that bit was
newer for some reason.