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

[Muchun Song]

On Mon, Feb 15, 2021 at 9:19 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Mon 15-02-21 20:44:57, Muchun Song wrote:
> > On Mon, Feb 15, 2021 at 8:18 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Mon 15-02-21 20:00:07, Muchun Song wrote:
> > > > On Mon, Feb 15, 2021 at 7:51 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Feb 15, 2021 at 6:33 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > > >
> > > > > > On Mon 15-02-21 18:05:06, Muchun Song wrote:
> > > > > > > On Fri, Feb 12, 2021 at 11:32 PM Michal Hocko <mhocko@xxxxxxxx> 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@xxxxxxxxxxxxx/
> > > > >
> > > > > > 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.
> >
> > I see you are worried that using GFP_ATOMIC will use reverse memory
> > unlimited. So I think that __GFP_NOMEMALLOC may be suitable for us.
> > Sorry, I may not understand the point you said. What I missed?
>
> OK, let me try to explain again. GFP_ATOMIC is not only a non-sleeping
> allocation request. It also grants access to memory reserves. The later
> is a bit more involved because there are more layers of memory reserves
> to access but that is not really important. Non-sleeping semantic can be
> achieved by GFP_NOWAIT which will not grant access to reserves unless
> explicitly stated - e.g. by __GFP_HIGH or __GFP_ATOMIC.
> Is that more clear?
>
> Now again why I do not think access to memory reserves is suitable.
> Hugetlb pages can be released in a large batches and that might cause a
> peak depletion of memory reserves which are normally used by other
> consumers as well. Other GFP_ATOMIC users might see allocation failures.
> Those shouldn't be really fatal as nobody should be relying on those and
> a failure usually mean a hand over to a different, less constrained,
> context. So this concern is more about a more well behaved behavior from
> the hugetlb side than a correctness.
> Is that more clear?

Ok. It is very clear. Very thanks for your patient explanations.

>
> There shouldn't be any real reason why the memory allocation for
> vmemmaps, or handling vmemmap in general, has to be done from within the
> hugetlb lock and therefore requiring a non-sleeping semantic. All that
> can be deferred to a more relaxed context. If you want to make a

Yeah, you are right. We can put the freeing hugetlb routine to a
workqueue. Just like I do in the previous version (before v13) patch.
I will pick up these patches.

> GFP_NOWAIT optimistic attempt in the direct free path then no problem
> but you have to expect failures under memory pressure. If you want to
> have a more robust allocation request then you have to go outside of the
> spin lock and use GFP_KERNEL | __GFP_NORETRY or GFP_KERNEL |
> __GFP_RETRY_MAYFAIL depending on how hard you want to try.
> __GFP_THISNODE makes a slight difference here but something that I would
> recommend not depending on.
> Is that more clear?

OK. I will use GFP_KERNEL instead of GFP_ATOMIC. Thanks for your
suggestions.


> --
> Michal Hocko
> SUSE Labs