linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Will Deacon <will.deacon@arm.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	kirill.shutemov@linux.intel.com, akpm@linux-foundation.org,
	mhocko@suse.com, zi.yan@cs.rutgers.edu
Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry
Date: Tue, 9 Oct 2018 14:18:04 +0100	[thread overview]
Message-ID: <20181009131803.GH6248@arm.com> (raw)
In-Reply-To: <20181009130421.bmus632ocurn275u@kshutemo-mobl1>

On Tue, Oct 09, 2018 at 04:04:21PM +0300, Kirill A. Shutemov wrote:
> On Tue, Oct 09, 2018 at 09:28:58AM +0530, Anshuman Khandual wrote:
> > A normal mapped THP page at PMD level should be correctly differentiated
> > from a PMD migration entry while walking the page table. A mapped THP would
> > additionally check positive for pmd_present() along with pmd_trans_huge()
> > as compared to a PMD migration entry. This just adds a new conditional test
> > differentiating the two while walking the page table.
> > 
> > Fixes: 616b8371539a6 ("mm: thp: enable thp migration in generic path")
> > Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> > ---
> > On X86, pmd_trans_huge() and is_pmd_migration_entry() are always mutually
> > exclusive which makes the current conditional block work for both mapped
> > and migration entries. This is not same with arm64 where pmd_trans_huge()
> > returns positive for both mapped and migration entries. Could some one
> > please explain why pmd_trans_huge() has to return false for migration
> > entries which just install swap bits and its still a PMD ?
> 
> I guess it's just a design choice. Any reason why arm64 cannot do the
> same?

Anshuman, would it work to:

#define pmd_trans_huge(pmd)     (pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT))

?

> > Nonetheless pmd_present() seems to be a better check to distinguish
> > between mapped and (non-mapped non-present) migration entries without
> > any ambiguity.
> 
> Can we instead reverse order of check:
> 
> if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) {
> 	pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> 	if (!pmd_present(*pvmw->pmd)) {
> 		...
> 	} else if (likely(pmd_trans_huge(*pvmw->pmd))) {
> 		...
> 	} else {
> 		...
> 	}
> ...
> 
> This should cover both imeplementations of pmd_trans_huge().

I'd much rather have portable semantics for pmd_trans_huge(), if we can
achieve that efficiently. But that would be fast /and/ correct, so perhaps
I'm being too hopeful :)

Will

  reply	other threads:[~2018-10-09 13:18 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-09  3:58 Anshuman Khandual
2018-10-09 13:04 ` Kirill A. Shutemov
2018-10-09 13:18   ` Will Deacon [this message]
2018-10-12  8:02     ` Anshuman Khandual
2018-10-15  8:32       ` Aneesh Kumar K.V
2018-10-16 13:16         ` Anshuman Khandual
2018-10-09 13:42   ` Anshuman Khandual
2018-10-09 13:58 ` Zi Yan
2018-10-10  4:05   ` Anshuman Khandual
2018-10-10 12:43     ` Zi Yan
2018-10-12  8:00       ` Anshuman Khandual
2018-10-15  0:53         ` Zi Yan
2018-10-15  4:06           ` Anshuman Khandual
2018-10-16 14:31             ` Zi Yan
2018-10-18  2:17               ` Naoya Horiguchi
2018-11-02  5:22                 ` Anshuman Khandual
2018-10-25  8:10               ` Anshuman Khandual
2018-10-25 18:45                 ` Zi Yan
2018-10-26  1:39                   ` Anshuman Khandual
2018-10-17  2:09           ` Andrea Arcangeli
2018-10-22 14:00             ` Zi Yan
2018-11-02  6:15             ` Anshuman Khandual
2018-11-06  0:35               ` Will Deacon
2018-11-06  9:51                 ` Anshuman Khandual

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20181009131803.GH6248@arm.com \
    --to=will.deacon@arm.com \
    --cc=akpm@linux-foundation.org \
    --cc=anshuman.khandual@arm.com \
    --cc=kirill.shutemov@linux.intel.com \
    --cc=kirill@shutemov.name \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@suse.com \
    --cc=zi.yan@cs.rutgers.edu \
    --subject='Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).