linux-kernel - Re: [External] Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

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Date:   Mon, 21 Dec 2020 19:00:19 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        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>, 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 v10 03/11] mm/hugetlb: Free the vmemmap
 pages associated with each HugeTLB page

On Mon, Dec 21, 2020 at 11:52:30PM +0800, Muchun Song wrote:
> On Mon, Dec 21, 2020 at 9:44 PM Oscar Salvador <osalvador@...e.de> wrote:
> >
> > On Mon, Dec 21, 2020 at 07:25:15PM +0800, Muchun Song wrote:
> >
> > > Should we add a BUG_ON in vmemmap_remap_free() for now?
> > >
> > >         BUG_ON(reuse != start + PAGE_SIZE);
> >
> > I do not think we have to, plus we would be BUG_ing for some specific use
> > case in "generic" function.
> 
> The vmemmap_remap_range() walks page table range [start, end),
> if reuse is equal to (start + PAGE_SIZE), the range can adjust to
> [start - PAGE_SIZE, end). But if not, we need some work to
> implement the "generic" function.
> 
>   - adjust range to [min(start, reuse), end) and call
>     vmemmap_remap_rangeand which skip the hole
>     which is [reuse + PAGE_SIZE, start) or [end, reuse).
>   - call vmemmap_remap_range(reuse, reuse + PAGE_SIZE)
>     to get the reuse page.Then, call vmemmap_remap_range(start, end)
>     again to remap.
> 
> Which one do you prefer?

I would not overcomplicate things at this stage.
Just follow my sugestion and add a BUG_ON as you said, that might be the
easier way now.
We can overthink this in the future when some other usecases come
around, right?

Thanks


-- 
Oscar Salvador
SUSE L3