[PATCH] mm/memory: Pass head page to do_set_pmd()

[PATCH] mm/memory: Pass head page to do_set_pmd()

[Andrew Bresticker]

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@rivosinc.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;
 		}
-- 
2.34.1

Re: [PATCH] mm/memory: Pass head page to do_set_pmd()

[David Hildenbrand]

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@rivosinc.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

Re: [PATCH] mm/memory: Pass head page to do_set_pmd()

[David Hildenbrand]

On 11.06.24 16:46, David Hildenbrand wrote:
> 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@rivosinc.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?

To be precise, the something like following:

diff --git a/mm/memory.c b/mm/memory.c
index 5e633c9c27a8b..76f48dc5899cd 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4699,8 +4699,9 @@ 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


-- 
Cheers,

David / dhildenb

Re: [PATCH] mm/memory: Pass head page to do_set_pmd()

[Andrew Bresticker]

On Tue, Jun 11, 2024 at 10:46 AM David Hildenbrand <david@redhat.com> wrote:
>
> 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@rivosinc.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.

thp_vma_suitable_order() checks that the folio fits in the vma, if
that's what you mean.

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

Indeed that's probably better as I'm now noticing that
filemap_map_pmd() has the same issue. Will send a v2.

Thanks.
-Andrew

>
> 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
>