linux-kernel - Re: [External] Re: [PATCH v15 4/8] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

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Date:   Mon, 15 Feb 2021 19:51:26 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Michal Hocko <mhocko@...e.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>,
        Oscar Salvador <osalvador@...e.de>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Joao Martins <joao.m.martins@...cle.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 v15 4/8] mm: hugetlb: alloc the vmemmap
 pages associated with each HugeTLB page

On Mon, Feb 15, 2021 at 6:33 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Mon 15-02-21 18:05:06, Muchun Song wrote:
> > On Fri, Feb 12, 2021 at 11:32 PM Michal Hocko <mhocko@...e.com> wrote:
> [...]
> > > > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > > > +{
> > > > +     int ret;
> > > > +     unsigned long vmemmap_addr = (unsigned long)head;
> > > > +     unsigned long vmemmap_end, vmemmap_reuse;
> > > > +
> > > > +     if (!free_vmemmap_pages_per_hpage(h))
> > > > +             return 0;
> > > > +
> > > > +     vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> > > > +     vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> > > > +     vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> > > > +
> > > > +     /*
> > > > +      * The pages which the vmemmap virtual address range [@vmemmap_addr,
> > > > +      * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> > > > +      * the range is mapped to the page which @vmemmap_reuse is mapped to.
> > > > +      * When a HugeTLB page is freed to the buddy allocator, previously
> > > > +      * discarded vmemmap pages must be allocated and remapping.
> > > > +      */
> > > > +     ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> > > > +                               GFP_ATOMIC | __GFP_NOWARN | __GFP_THISNODE);
> > >
> > > I do not think that this is a good allocation mode. GFP_ATOMIC is a non
> > > sleeping allocation and a medium memory pressure might cause it to
> > > fail prematurely. I do not think this is really an atomic context which
> > > couldn't afford memory reclaim. I also do not think we want to grant
> >
> > Because alloc_huge_page_vmemmap is called under hugetlb_lock
> > now. So using GFP_ATOMIC indeed makes the code more simpler.
>
> You can have a preallocated list of pages prior taking the lock.

A discussion about this can refer to here:

https://patchwork.kernel.org/project/linux-mm/patch/20210117151053.24600-5-songmuchun@bytedance.com/

> Moreover do we want to manipulate vmemmaps from under spinlock in
> general. I have to say I have missed that detail when reviewing. Need to
> think more.
>
> > From the document of the kernel, I learned that __GFP_NOMEMALLOC
> > can be used to explicitly forbid access to emergency reserves. So if
> > we do not want to use the reserve memory. How about replacing it to
> >
> > GFP_ATOMIC | __GFP_NOMEMALLOC | __GFP_NOWARN | __GFP_THISNODE
>
> The whole point of GFP_ATOMIC is to grant access to memory reserves so
> the above is quite dubious. If you do not want access to memory reserves

Look at the code of gfp_to_alloc_flags().

static inline unsigned int gfp_to_alloc_flags(gfp_t gfp_mask)
{
        [...]
        if (gfp_mask & __GFP_ATOMIC) {
        /*
         * Not worth trying to allocate harder for __GFP_NOMEMALLOC even
         * if it can't schedule.
         */
        if (!(gfp_mask & __GFP_NOMEMALLOC))
                alloc_flags |= ALLOC_HARDER;
       [...]
}

Seems to allow this operation (GFP_ATOMIC | __GFP_NOMEMALLOC).

> then use GFP_NOWAIT instead. But failures are much more easier to happen
> then.
>
> NOMEMALLOC is meant to be used from paths which are allowed to consume
> memory reserves - e.g. when invoked from the memory reclaim path.
> --
> Michal Hocko
> SUSE Labs