linux-kernel - Re: [PATCH v2] mm/memory: Don't require head page for do_set

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Message-ID: <9419ce11-08b5-4f80-9dc6-8551a7b8daea@redhat.com>
Date: Wed, 12 Jun 2024 20:41:47 +0200
From: David Hildenbrand <david@...hat.com>
To: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Matthew Wilcox <willy@...radead.org>,
 Andrew Bresticker <abrestic@...osinc.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/memory: Don't require head page for do_set_pmd()

On 11.06.24 23:18, Hugh Dickins wrote:
> On Tue, 11 Jun 2024, Andrew Morton wrote:
>> On Tue, 11 Jun 2024 19:22:03 +0100 Matthew Wilcox <willy@...radead.org> wrote:
>>> On Tue, Jun 11, 2024 at 11:06:22AM -0700, Andrew Morton wrote:
>>>> On Tue, 11 Jun 2024 17:33:17 +0200 David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>>> On 11.06.24 17:32, Andrew Bresticker wrote:
>>>>>> The requirement that the head page be passed to do_set_pmd() was added
>>>>>> in commit ef37b2ea08ac ("mm/memory: page_add_file_rmap() ->
>>>>>> folio_add_file_rmap_[pte|pmd]()") and prevents pmd-mapping in the
>>>>>> finish_fault() and filemap_map_pages() paths if the page to be inserted
>>>>>> is anything but the head page for an otherwise suitable vma and pmd-sized
>>>>>> page.
>>>>>>
>>>>>> Fixes: ef37b2ea08ac ("mm/memory: page_add_file_rmap() -> folio_add_file_rmap_[pte|pmd]()")
>>>>>> Signed-off-by: Andrew Bresticker <abrestic@...osinc.com>
>>>>>> ---
>>>>>>    mm/memory.c | 3 ++-
>>>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>>>> index 0f47a533014e..a1fce5ddacb3 100644
>>>>>> --- a/mm/memory.c
>>>>>> +++ b/mm/memory.c
>>>>>> @@ -4614,8 +4614,9 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
>>>>>>    	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
>>>>>>    		return ret;
>>>>>>    
>>>>>> -	if (page != &folio->page || folio_order(folio) != HPAGE_PMD_ORDER)
>>>>>> +	if (folio_order(folio) != HPAGE_PMD_ORDER)
>>>>>>    		return ret;
>>>>>> +	page = &folio->page;
>>>>>>    
>>>>>>    	/*
>>>>>>    	 * Just backoff if any subpage of a THP is corrupted otherwise
>>>>>
>>>>> Acked-by: David Hildenbrand <david@...hat.com>
> 
> Acked-by: Hugh Dickins <hughd@...gle.com>
> 
>>>>
>>>> You know what I'm going to ask ;) I'm assuming that the runtime effects
>>>> are "small performance optimization" and that "should we backport the
>>>> fix" is "no".
>>>
>>> We're going to stop using PMDs to map large folios unless the fault is
>>> within the first 4KiB of the PMD.  No idea how many workloads that
>>> affects, but it only needs to be backported as far as v6.8, so we
>>> may as well backport it.
>>
>> OK, thanks, I pasted the above text and added the cc:stable.
> 
> Yes please. My interest in this being that yesterday I discovered
> the large drop in ShmemPmdMapped between v6.7 and v6.8, bisected,
> and was testing overnight with a patch very much like this one of
> Andrew's. I'd been hoping to send mine today, but now no need.
> 
>>
>> I didn't move it into the hotfixes queue - it's a non-trivial
>> behavioral change and extra test time seems prudent(?).
> 
> It is certainly worth some test soak time, and the bug might have
> been masking other issues which may now emerge; but the fix is
> just reverting to the old pre-v6.8 behaviour.

Right, I don't expect surprises, really. I'm rather surprised that 
nobody noticed and that the usual 0-day benchmarks don't trigger that case.

-- 
Cheers,

David / dhildenb