linux-kernel - Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared pages

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Message-ID: <20180823193600.GV29735@dhcp22.suse.cz>
Date:   Thu, 23 Aug 2018 21:36:00 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: migration: fix migration of huge PMD shared
 pages

On Thu 23-08-18 10:56:30, Mike Kravetz wrote:
> On 08/23/2018 10:01 AM, Mike Kravetz wrote:
> > On 08/23/2018 05:48 AM, Michal Hocko wrote:
> >> On Tue 21-08-18 18:10:42, Mike Kravetz wrote:
> >> [...]
> >>
> >> OK, after burning myself when trying to be clever here it seems like
> >> your proposed solution is indeed simpler.
> >>
> >>> +bool huge_pmd_sharing_possible(struct vm_area_struct *vma,
> >>> +				unsigned long *start, unsigned long *end)
> >>> +{
> >>> +	unsigned long check_addr = *start;
> >>> +	bool ret = false;
> >>> +
> >>> +	if (!(vma->vm_flags & VM_MAYSHARE))
> >>> +		return ret;
> >>> +
> >>> +	for (check_addr = *start; check_addr < *end; check_addr += PUD_SIZE) {
> >>> +		unsigned long a_start = check_addr & PUD_MASK;
> >>> +		unsigned long a_end = a_start + PUD_SIZE;
> >>
> >> I guess this should be rather in HPAGE_SIZE * PTRS_PER_PTE units as
> >> huge_pmd_unshare does.
> > 
> > Sure, I can do that.
> 
> On second thought, this is more similar to vma_shareable() which uses
> PUD_MASK and PUD_SIZE.  In fact Kirill asked me to put in a common helper
> for this and vma_shareable.  So, I would prefer to leave it as PUD* unless
> you feel strongly.

I don't
 
> IMO, it would make more sense to change this in huge_pmd_unshare as PMD
> sharing is pretty explicitly tied to PUD_SIZE.  But, that is a future cleanup.

Fair enough. I didn't realize we are PUD explicit elsewhere. So if you
plan to update huge_pmd_unshare to be in sync then no objections from me
at all. I merely wanted to be in sync with this because it is a central
point to look at wrt pmd sharing.
-- 
Michal Hocko
SUSE Labs