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Date:   Mon, 15 Feb 2021 13:18:28 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>, mingo@redhat.com,
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        Peter Zijlstra <peterz@infradead.org>, viro@zeniv.linux.org.uk,
        Andrew Morton <akpm@linux-foundation.org>, paulmck@kernel.org,
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        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Oscar Salvador <osalvador@suse.de>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>,
        David Hildenbrand <david@redhat.com>,
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Subject: Re: [External] Re: [PATCH v15 4/8] mm: hugetlb: alloc the vmemmap
 pages associated with each HugeTLB page
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On Mon 15-02-21 20:00:07, Muchun Song wrote:
> On Mon, Feb 15, 2021 at 7:51 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > On Mon, Feb 15, 2021 at 6:33 PM Michal Hocko <mhocko@suse.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@suse.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).

Please read my response again more carefully. I am not claiming that
combination is not allowed. I have said it doesn't make any sense in
this context.

-- 
Michal Hocko
SUSE Labs