linux-kernel - Re: [External] Re: [PATCH v12 04/13] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

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Date:   Thu, 14 Jan 2021 18:54:30 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Oscar Salvador <osalvador@...e.de>
Cc:     Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, viro@...iv.linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v12 04/13] mm/hugetlb: Free the vmemmap
 pages associated with each HugeTLB page

On Thu, Jan 14, 2021 at 7:27 AM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 1/13/21 1:20 AM, Oscar Salvador wrote:
> > On Tue, Jan 12, 2021 at 07:33:33PM +0800, Muchun Song wrote:
> >>> It seems a bit odd to only pass "start" for the BUG_ON.
> >>> Also, I kind of dislike the "addr += PAGE_SIZE" in vmemmap_pte_range.
> >>>
> >>> I wonder if adding a ".remap_start_addr" would make more sense.
> >>> And adding it here with the vmemmap_remap_walk init.
> >>
> >> How about introducing a new function which aims to get the reuse
> >> page? In this case, we can drop the BUG_ON() and "addr += PAGE_SIZE"
> >> which is in vmemmap_pte_range. The vmemmap_remap_range only
> >> does the remapping.
> >
> > How would that look?
> > It might be good, dunno, but the point is, we should try to make the rules as
> > simple as possible, dropping weird assumptions.
> >
> > Callers of vmemmap_remap_free should know three things:
> >
> > - Range to be remapped
> > - Addr to remap to
> > - Current implemantion needs addr to be remap to to be part of the complete
> >   range
> >
> > right?
>
> And, current implementation needs must have remap addr be the first in the
> complete range.  This is just because of the way the page tables are walked
> for remapping.  The remap/reuse page must be found first so that the following
> pages can be remapped to it.

You are right.

>
> That implementation seems to be the 'most efficient' for hugetlb pages where
> we want vmemmap pages n+3 and beyond mapped to n+2.
>
> In a more general purpose vmemmap_remap_free implementation, the reuse/remap
> address would not necessarily need to be related to the range.  However, this
> would require a separate page table walk/validation for the reuse address
> independent of the range.  This may be what Muchun was proposing for 'a new
> function which aims to get the reuse page'.

Agree.


>
> IMO, the decision on how to implement depends on the intended use case.
> - If this is going to be hugetlb only (or perhaps generic huge page only)
>   functionality, then I am OK with an efficient implementation that has
>   some restrictions.
> - If we see this being used for more general purpose remapping, then we
>   should go with a more general purpose implementation.

I think this approach may be only suitable for generic huge page only.
So we can implement it only for huge page.

Hi Oscar,

What's your opinion about this?

>
> Again, just my opinion.
> --
> Mike Kravetz