linux-kernel - Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio

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Date: Wed,  5 Jun 2024 02:20:51 +0000
From: ran xiaokai <ranxiaokai627@....com>
To: david@...hat.com
Cc: akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	mhocko@...nel.org,
	v-songbaohua@...o.com,
	ran.xiaokai@....com.cn,
	xu.xin16@....com.cn,
	yang.yang29@....com.cn,
	ziy@...dia.com
Subject: Re: [PATCH linux-next] mm: huge_memory: fix misused mapping_large_folio_support() for anon folios

> On 04.06.24 07:47, xu.xin16@....com.cn wrote:
> > From: Ran Xiaokai <ran.xiaokai@....com.cn>
> > 
> > When I did a large folios split test, a WARNING
> > "[ 5059.122759][  T166] Cannot split file folio to non-0 order"
> > was triggered. But my test cases are only for anonmous folios.
> > while mapping_large_folio_support() is only reasonable for page
> > cache folios.
> 
> Agreed.
> 
> I wonder if mapping_large_folio_support() should either
> 
> a) Complain if used for anon folios, so we can detect the wrong use more 
> easily. (VM_WARN_ON_ONCE())

> b) Return "true" for anonymous mappings, although that's more debatable.
> 

Hi, David,
Thanks for the review.
I think a) is better. 
But we have to add a new parameter "folio" to mapping_large_folio_support(), right ?
something like mapping_large_folio_support(struct address_space *mapping, struct folio *folio) ?
But in the __filemap_get_folio() path, 

__filemap_get_folio()
  no_page:
    ....
    if (!mapping_large_folio_support(mapping))

the folio is not allocated yet, yes ?
Or do you mean there is some other way to do this ? 

> > 
> > In split_huge_page_to_list_to_order(), the folio passed to
> > mapping_large_folio_support() maybe anonmous folio. The
> > folio_test_anon() check is missing. So the split of the anonmous THP
> > is failed. This is also the same for shmem_mapping(). We'd better add
> > a check for both. But the shmem_mapping() in __split_huge_page() is
> > not involved, as for anonmous folios, the end parameter is set to -1, so
> > (head[i].index >= end) is always false. shmem_mapping() is not called.
> > 
> > Using /sys/kernel/debug/split_huge_pages to verify this, with this
> > patch, large anon THP is successfully split and the warning is ceased.
> > 
> > Signed-off-by: Ran Xiaokai <ran.xiaokai@....com.cn>
> > Cc: xu xin <xu.xin16@....com.cn>
> > Cc: Yang Yang <yang.yang29@....com.cn>
> > ---
> >   mm/huge_memory.c | 38 ++++++++++++++++++++------------------
> >   1 file changed, 20 insertions(+), 18 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 317de2afd371..4c9c7e5ea20c 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3009,31 +3009,33 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >   	if (new_order >= folio_order(folio))
> >   		return -EINVAL;
> > 
> > -	/* Cannot split anonymous THP to order-1 */
> > -	if (new_order == 1 && folio_test_anon(folio)) {
> > -		VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> > -		return -EINVAL;
> > -	}
> > -
> >   	if (new_order) {
> >   		/* Only swapping a whole PMD-mapped folio is supported */
> >   		if (folio_test_swapcache(folio))
> >   			return -EINVAL;
> > -		/* Split shmem folio to non-zero order not supported */
> > -		if (shmem_mapping(folio->mapping)) {
> > -			VM_WARN_ONCE(1,
> > -				"Cannot split shmem folio to non-0 order");
> > -			return -EINVAL;
> > -		}
> > -		/* No split if the file system does not support large folio */
> > -		if (!mapping_large_folio_support(folio->mapping)) {
> > -			VM_WARN_ONCE(1,
> > -				"Cannot split file folio to non-0 order");
> > -			return -EINVAL;
> > +
> > +		if (folio_test_anon(folio)) {
> > +			/* Cannot split anonymous THP to order-1 */
> > +			if (new_order == 1) {
> > +				VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> > +				return -EINVAL;
> > +			}
> > +		} else {
> > +			/* Split shmem folio to non-zero order not supported */
> > +			if (shmem_mapping(folio->mapping)) {
> > +				VM_WARN_ONCE(1,
> > +					"Cannot split shmem folio to non-0 order");
> > +				return -EINVAL;
> > +			}
> > +			/* No split if the file system does not support large folio */
> > +			if (!mapping_large_folio_support(folio->mapping)) {
> > +				VM_WARN_ONCE(1,
> > +					"Cannot split file folio to non-0 order");
> > +				return -EINVAL;
> > +			}
> >   		}
> >   	}
> 
> What about the following sequence:
> 
> if (folio_test_anon(folio)) {
> 	if (new_order == 1)
> 		...
> } else if (new_order) {
> 	if (shmem_mapping(...))
> 		...
> 	...
> }
> 
> if (folio_test_swapcache(folio) && new_order)
> 	return -EINVAL;
> 
> Should result in less churn and reduce indentation level.
>

Thanks.
The code is cleaner in this way.
 
> -- 
> Cheers,
> 
> David / dhildenb