linux-kernel - Re: [PATCH] mm/memory: Pass head page to do_set

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Message-ID: <6b5392f5-209d-4d3e-b5c3-ba875bf8416d@redhat.com>
Date: Tue, 11 Jun 2024 16:46:00 +0200
From: David Hildenbrand <david@...hat.com>
To: Andrew Bresticker <abrestic@...osinc.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH] mm/memory: Pass head page to do_set_pmd()

On 11.06.24 16:18, 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() path if vmf->page is anything but the head page for an
> otherwise suitable vma and pmd-sized page. Have finish_fault() pass in
> the head page instead.
> 
> 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 | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 0f47a533014e..f13b953b507c 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4764,7 +4764,7 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   
>   	if (pmd_none(*vmf->pmd)) {
>   		if (PageTransCompound(page)) {
> -			ret = do_set_pmd(vmf, page);
> +			ret = do_set_pmd(vmf, compound_head(page));
>   			if (ret != VM_FAULT_FALLBACK)
>   				return ret;
>   		}

That certainly makes the "page != &folio->page" check happy.

It is *likely* incorrect if we would ever have folios > PMD size (which 
we don't have on that path yet).

I assume that the thp_vma_suitable_order() check would detect any kind 
of "different placement of the folio in virtual address space", where we 
could mess up.

Question is: should we instead drop the "page != &folio->page" check 
that I added?

I think I added that check because I saw the "compound_order(page)" 
check and assumed it would return 0 for tail pages, but missed that we 
get the compound head first.

-- 
Cheers,

David / dhildenb