Re: [PATCH 3/4] mm/thp: allow drop THP from page cache

[Song Liu]

> On Oct 17, 2019, at 9:12 AM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> 
> On Wed, Oct 16, 2019 at 12:37:30AM -0700, Song Liu wrote:
>> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>> 
>> Once a THP is added to the page cache, it cannot be dropped via
>> /proc/sys/vm/drop_caches. Fix this issue with proper handling in
>> invalidate_mapping_pages() and __remove_mapping().
>> 
>> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
>> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
>> Tested-by: Song Liu <songliubraving@xxxxxx>
>> Signed-off-by: Song Liu <songliubraving@xxxxxx>
>> ---
>> mm/truncate.c | 12 ++++++++++++
>> mm/vmscan.c   |  3 ++-
>> 2 files changed, 14 insertions(+), 1 deletion(-)
>> 
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index c6659bb758a4..1d80a188ad4a 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -932,7 +932,8 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
>> 	 * Note that if SetPageDirty is always performed via set_page_dirty,
>> 	 * and thus under the i_pages lock, then this ordering is not required.
>> 	 */
>> -	if (unlikely(PageTransHuge(page)) && PageSwapCache(page))
>> +	if (unlikely(PageTransHuge(page)) &&
>> +			(PageSwapCache(page) || !PageSwapBacked(page)))
>> 		refcount = 1 + HPAGE_PMD_NR;
>> 	else
>> 		refcount = 2;
> 
> Kirill suggests that this patch would be better (for this part of the patch;
> the part in truncate.c should remain as it is)
> 
> commit ddcee327f96d57cb9d5310486d21e43892b7a368
> Author: William Kucharski <william.kucharski@xxxxxxxxxx>
> Date:   Fri Sep 20 16:14:51 2019 -0400
> 
>    mm: Support removing arbitrary sized pages from mapping
> 
>    __remove_mapping() assumes that pages can only be either base pages
>    or HPAGE_PMD_SIZE.  Further, it assumes that large pages are
>    swap-backed.  Support all kinds of pages by unconditionally asking how
>    many pages this page references.
> 
>    Signed-off-by: William Kucharski <william.kucharski@xxxxxxxxxx>
>    Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c6659bb758a4..f870da1f4bb7 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -932,10 +932,7 @@ static int __remove_mapping(struct address_space *mapping, struct page *page,
> 	 * Note that if SetPageDirty is always performed via set_page_dirty,
> 	 * and thus under the i_pages lock, then this ordering is not required.
> 	 */
> -	if (unlikely(PageTransHuge(page)) && PageSwapCache(page))
> -		refcount = 1 + HPAGE_PMD_NR;
> -	else
> -		refcount = 2;
> +	refcount = 1 + compound_nr(page);
> 	if (!page_ref_freeze(page, refcount))
> 		goto cannot_free;
> 	/* note: atomic_cmpxchg in page_ref_freeze provides the smp_rmb */

This does look cleaner, and works fine in my tests.

Let me include it in v2 set. 

Thanks,
Song