linux-kernel - Re: [PATCH v2] mm,thp: recheck each page before collapsing file THP

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Message-ID: <696F8683-9A4B-4CD7-A96A-1E68E6A92E02@fb.com>
Date:   Fri, 18 Oct 2019 17:27:12 +0000
From:   Song Liu <songliubraving@...com>
To:     Johannes Weiner <hannes@...xchg.org>
CC:     open list <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        Kernel Team <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Hugh Dickins" <hughd@...gle.com>
Subject: Re: [PATCH v2] mm,thp: recheck each page before collapsing file THP



> On Oct 18, 2019, at 9:58 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> 
> On Fri, Oct 18, 2019 at 12:49:46PM -0400, Johannes Weiner wrote:
>> On Fri, Oct 18, 2019 at 09:37:54AM -0700, Song Liu wrote:
>>> In collapse_file(), after locking the page, it is necessary to recheck
>>> that the page is up-to-date. Add PageUptodate() check for both shmem THP
>>> and file THP.
>>> 
>>> Current khugepaged should not try to collapse dirty file THP, because it
>>> is limited to read only text. Add a PageDirty check and warning for file
>>> THP. This is added after page_mapping() check, because if the page is
>>> truncated, it might be dirty.
>>> 
>>> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
>>> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>>> Cc: Johannes Weiner <hannes@...xchg.org>
>>> Cc: Hugh Dickins <hughd@...gle.com>
>>> Cc: William Kucharski <william.kucharski@...cle.com>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Signed-off-by: Song Liu <songliubraving@...com>
>> 
>> Acked-by: Johannes Weiner <hannes@...xchg.org>
> 
> We should also be able to remove the unlocked tests for those two
> conditions, right?

Yeah, I think we can remove them. 

Thanks,
Song

> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 0a1b4b484ac5..a3ef6ce86bfa 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1601,17 +1601,6 @@ static void collapse_file(struct mm_struct *mm,
> 					result = SCAN_FAIL;
> 					goto xa_unlocked;
> 				}
> -			} else if (!PageUptodate(page)) {
> -				xas_unlock_irq(&xas);
> -				wait_on_page_locked(page);
> -				if (!trylock_page(page)) {
> -					result = SCAN_PAGE_LOCK;
> -					goto xa_unlocked;
> -				}
> -				get_page(page);
> -			} else if (PageDirty(page)) {
> -				result = SCAN_FAIL;
> -				goto xa_locked;
> 			} else if (trylock_page(page)) {
> 				get_page(page);
> 				xas_unlock_irq(&xas);