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

[Ryan Roberts]

On 25/06/2024 15:06, Matthew Wilcox wrote:
> On Tue, Jun 25, 2024 at 02:41:18PM +0100, Ryan Roberts wrote:
>> 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.
> 
> The pagecache doesn't know whether the file contains data or holes.
> It allocates folios and then invites the filesystem to fill them; the
> filesystem checks its data structures and then either issues reads if
> there's data on media or calls memset if the records indicate there's
> a hole.
> 
> Whether it chooses to allocate large folios or not is going to depend
> on the access pattern; a sequential write pattern will use large folios
> and a random write pattern won't.
> 
> Now, I've oversimplified things a bit by talking about filemap_fault.
> Before we call filemap_fault, we call filemap_map_pages() which looks
> for any suitable folios in the page cache between start and end, and
> maps those.

OK that all makes sense, thanks. I guess it just means I don't have an excuse
for the perf regression. :)