linux-kernel - Re: [PATCH 0/1] memory offline issues with hugepage size

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Date:   Wed, 21 Sep 2016 12:30:35 +0200
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Rui Teng <rui.teng@...ux.vnet.ibm.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 0/1] memory offline issues with hugepage size > memory
 block size

On Tue, 20 Sep 2016 10:37:04 -0700
Mike Kravetz <mike.kravetz@...cle.com> wrote:

> 
> Cc'ed Rui Teng and Dave Hansen as they were discussing the issue in
> this thread:
> https://lkml.org/lkml/2016/9/13/146

Ah, thanks, I didn't see that.

> 
> Their approach (I believe) would be to fail the offline operation in
> this case.  However, I could argue that failing the operation, or
> dissolving the unused huge page containing the area to be offlined is
> the right thing to do.
> 
> I never thought too much about the VM_BUG_ON(), but you are correct in
> that it should be removed in either case.
> 
> The other thing that needs to be changed is the locking in
> dissolve_free_huge_page().  I believe the lock only needs to be held if
> we are removing the huge page from the pool.  It is not a correctness
> but performance issue.
> 

Yes, that looks odd, that's why in my patch I moved the PageHuge() check
out from dissolve_free_huge_page(), up into the loop in
dissolve_free_huge_pages(). This way dissolve_free_huge_page() with its
locking should only be called once per memory block, in the case where
this memory block is part of a gigantic hugepage.