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SubjectRe: [PATCH 2/2] mm, thp: consolidate THP gfp handling into alloc_hugepage_direct_gfpmask
On Wed 26-09-18 16:30:39, Kirill A. Shutemov wrote:
> On Tue, Sep 25, 2018 at 02:03:26PM +0200, Michal Hocko wrote:
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index c3bc7e9c9a2a..c0bcede31930 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -629,21 +629,40 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
> > * available
> > * never: never stall for any thp allocation
> > */
> > -static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
> > +static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma, unsigned long addr)
> > {
> > const bool vma_madvised = !!(vma->vm_flags & VM_HUGEPAGE);
> > + gfp_t this_node = 0;
> > +
> > +#ifdef CONFIG_NUMA
> > + struct mempolicy *pol;
> > + /*
> > + * __GFP_THISNODE is used only when __GFP_DIRECT_RECLAIM is not
> > + * specified, to express a general desire to stay on the current
> > + * node for optimistic allocation attempts. If the defrag mode
> > + * and/or madvise hint requires the direct reclaim then we prefer
> > + * to fallback to other node rather than node reclaim because that
> > + * can lead to excessive reclaim even though there is free memory
> > + * on other nodes. We expect that NUMA preferences are specified
> > + * by memory policies.
> > + */
> > + pol = get_vma_policy(vma, addr);
> > + if (pol->mode != MPOL_BIND)
> > + this_node = __GFP_THISNODE;
> > + mpol_cond_put(pol);
> > +#endif
>
> I'm not very good with NUMA policies. Could you explain in more details how
> the code above is equivalent to the code below?

MPOL_PREFERRED is handled by policy_node() before we call __alloc_pages_nodemask.
__GFP_THISNODE is applied only when we are not using
__GFP_DIRECT_RECLAIM which is handled in alloc_hugepage_direct_gfpmask
now.
Lastly MPOL_BIND wasn't handled explicitly but in the end the removed
late check would remove __GFP_THISNODE for it as well. So in the end we
are doing the same thing unless I miss something

> > @@ -2026,60 +2025,6 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
> > goto out;
> > }
> >
> > - if (unlikely(IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && hugepage)) {
> > - int hpage_node = node;
> > -
> > - /*
> > - * For hugepage allocation and non-interleave policy which
> > - * allows the current node (or other explicitly preferred
> > - * node) we only try to allocate from the current/preferred
> > - * node and don't fall back to other nodes, as the cost of
> > - * remote accesses would likely offset THP benefits.
> > - *
> > - * If the policy is interleave, or does not allow the current
> > - * node in its nodemask, we allocate the standard way.
> > - */
> > - if (pol->mode == MPOL_PREFERRED &&
> > - !(pol->flags & MPOL_F_LOCAL))
> > - hpage_node = pol->v.preferred_node;
> > -
> > - nmask = policy_nodemask(gfp, pol);
> > - if (!nmask || node_isset(hpage_node, *nmask)) {
> > - mpol_cond_put(pol);
> > - /*
> > - * We cannot invoke reclaim if __GFP_THISNODE
> > - * is set. Invoking reclaim with
> > - * __GFP_THISNODE set, would cause THP
> > - * allocations to trigger heavy swapping
> > - * despite there may be tons of free memory
> > - * (including potentially plenty of THP
> > - * already available in the buddy) on all the
> > - * other NUMA nodes.
> > - *
> > - * At most we could invoke compaction when
> > - * __GFP_THISNODE is set (but we would need to
> > - * refrain from invoking reclaim even if
> > - * compaction returned COMPACT_SKIPPED because
> > - * there wasn't not enough memory to succeed
> > - * compaction). For now just avoid
> > - * __GFP_THISNODE instead of limiting the
> > - * allocation path to a strict and single
> > - * compaction invocation.
> > - *
> > - * Supposedly if direct reclaim was enabled by
> > - * the caller, the app prefers THP regardless
> > - * of the node it comes from so this would be
> > - * more desiderable behavior than only
> > - * providing THP originated from the local
> > - * node in such case.
> > - */
> > - if (!(gfp & __GFP_DIRECT_RECLAIM))
> > - gfp |= __GFP_THISNODE;
> > - page = __alloc_pages_node(hpage_node, gfp, order);
> > - goto out;
> > - }
> > - }
> > -
> > nmask = policy_nodemask(gfp, pol);
> > preferred_nid = policy_node(gfp, pol, node);
> > page = __alloc_pages_nodemask(gfp, order, preferred_nid, nmask);
>
> --
> Kirill A. Shutemov

--
Michal Hocko
SUSE Labs

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