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    SubjectRe: [PATCH] mm/page_alloc: Make alloc_gigantic_page() available for general use
    On Wed 16-10-19 10:56:16, David Hildenbrand wrote:
    > On 16.10.19 10:51, Michal Hocko wrote:
    > > On Wed 16-10-19 10:08:21, David Hildenbrand wrote:
    > > > On 16.10.19 09:34, Anshuman Khandual wrote:
    > > [...]
    > > > > +static bool pfn_range_valid_contig(struct zone *z, unsigned long start_pfn,
    > > > > + unsigned long nr_pages)
    > > > > +{
    > > > > + unsigned long i, end_pfn = start_pfn + nr_pages;
    > > > > + struct page *page;
    > > > > +
    > > > > + for (i = start_pfn; i < end_pfn; i++) {
    > > > > + page = pfn_to_online_page(i);
    > > > > + if (!page)
    > > > > + return false;
    > > > > +
    > > > > + if (page_zone(page) != z)
    > > > > + return false;
    > > > > +
    > > > > + if (PageReserved(page))
    > > > > + return false;
    > > > > +
    > > > > + if (page_count(page) > 0)
    > > > > + return false;
    > > > > +
    > > > > + if (PageHuge(page))
    > > > > + return false;
    > > > > + }
    > > >
    > > > We might still try to allocate a lot of ranges that contain unmovable data
    > > > (we could avoid isolating a lot of page blocks in the first place). I'd love
    > > > to see something like pfn_range_movable() (similar, but different to
    > > > is_mem_section_removable(), which uses has_unmovable_pages()).
    > >
    > > Just to make sure I understand. Do you want has_unmovable_pages to be
    > > called inside pfn_range_valid_contig?
    >
    > I think this requires more thought, as has_unmovable_pages() works on
    > pageblocks only AFAIK. If you try to allocate < MAX_ORDER - 1, you could get
    > a lot of false positives.
    >
    > E.g., if a free "MAX_ORDER - 1" page spans two pageblocks and you only test
    > the second pageblock, you might detect "unmovable" if not taking proper care
    > of the "bigger" free page. (alloc_contig_range() properly works around that
    > issue)

    OK, I see your point. You are right that false positives are possible. I
    would deal with those in a separate patch though.

    > > [...]
    > > > > +struct page *alloc_contig_pages(unsigned long nr_pages, gfp_t gfp_mask,
    > > > > + int nid, nodemask_t *nodemask)
    > > > > +{
    > > > > + unsigned long ret, pfn, flags;
    > > > > + struct zonelist *zonelist;
    > > > > + struct zone *zone;
    > > > > + struct zoneref *z;
    > > > > +
    > > > > + zonelist = node_zonelist(nid, gfp_mask);
    > > > > + for_each_zone_zonelist_nodemask(zone, z, zonelist,
    > > > > + gfp_zone(gfp_mask), nodemask) {
    > > >
    > > > One important part is to never use the MOVABLE zone here (otherwise
    > > > unmovable data would end up on the movable zone). But I guess the caller is
    > > > responsible for that (not pass GFP_MOVABLE) like gigantic pages do.
    > >
    > > Well, if the caller uses GFP_MOVABLE then the movability should be
    > > implemented in some form. If that is not the case then it is a bug on
    > > the caller behalf.
    > >
    > > > > + spin_lock_irqsave(&zone->lock, flags);
    > > > > +
    > > > > + pfn = ALIGN(zone->zone_start_pfn, nr_pages);
    > > >
    > > > This alignment does not make too much sense when allowing passing in !power
    > > > of two orders. Maybe the caller should specify the requested alignment
    > > > instead? Or should we enforce this to be aligned to make our life easier for
    > > > now?
    > >
    > > Are there any usecases that would require than the page alignment?
    >
    > Gigantic pages have to be aligned AFAIK.

    Aligned to what? I do not see any guarantee like that in the existing
    code.

    --
    Michal Hocko
    SUSE Labs

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