Re: [PATCH v3 03/12] mm/khugepaged: make hugepage allocation context-specific

["Zach O'Keefe" <zokeefe@google.com>]

On Wed, Apr 27, 2022 at 1:05 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Apr 26, 2022 at 07:44:03AM -0700, Zach O'Keefe wrote:
> > Add hugepage allocation context to struct collapse_context, allowing
> > different collapse contexts to allocate hugepages differently.  For
> > example, khugepaged decides to allocate differently in NUMA and UMA
> > configurations, and other collapse contexts shouldn't be coupled to this
> > decision.  Likewise for the gfp flags used for said allocation.
> >
> > Additionally, move [pre]allocated hugepage pointer into
> > struct collapse_context.
> >
> > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > Reported-by: kernel test robot <lkp@intel.com>
>
> (Please remember to remove this one too, and the rest ones.. :)
>

Done :) Thanks

> > ---
> >  mm/khugepaged.c | 102 ++++++++++++++++++++++++------------------------
> >  1 file changed, 52 insertions(+), 50 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 9d42fa330812..c4962191d6e1 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -92,6 +92,11 @@ struct collapse_control {
> >
> >       /* Last target selected in khugepaged_find_target_node() for this scan */
> >       int last_target_node;
> > +
> > +     struct page *hpage;
> > +     gfp_t (*gfp)(void);
> > +     struct page* (*alloc_hpage)(struct collapse_control *cc, gfp_t gfp,
> > +                                 int node);
>
> Nit: s/page* /page */ looks a bit more consistent..
>

Done - thanks for catching.

> >  };
> >
> >  /**
> > @@ -877,21 +882,21 @@ static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
> >       return true;
> >  }
> >
> > -static struct page *
> > -khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
> > +static struct page *khugepaged_alloc_page(struct collapse_control *cc,
> > +                                       gfp_t gfp, int node)
>
> Nit: I'd suggest we put collapse_control* either at the start or at the
> end, and keep doing it when possible.
>
> IIRC you used to put it always at the end, but now it's at the start.  Not
> a big deal but it'll be nice to keep the code self-aligned.
>

Makes sense to me to be consistent. I'll move it to the end here and
look over the other patches to make an effort to do the same. General
strategy was to keep "output" arguments at the very end.

> >  {
> > -     VM_BUG_ON_PAGE(*hpage, *hpage);
> > +     VM_BUG_ON_PAGE(cc->hpage, cc->hpage);
> >
> > -     *hpage = __alloc_pages_node(node, gfp, HPAGE_PMD_ORDER);
> > -     if (unlikely(!*hpage)) {
> > +     cc->hpage = __alloc_pages_node(node, gfp, HPAGE_PMD_ORDER);
> > +     if (unlikely(!cc->hpage)) {
> >               count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
> > -             *hpage = ERR_PTR(-ENOMEM);
> > +             cc->hpage = ERR_PTR(-ENOMEM);
> >               return NULL;
> >       }
> >
> > -     prep_transhuge_page(*hpage);
> > +     prep_transhuge_page(cc->hpage);
> >       count_vm_event(THP_COLLAPSE_ALLOC);
> > -     return *hpage;
> > +     return cc->hpage;
> >  }
> >  #else
> >  static int khugepaged_find_target_node(struct collapse_control *cc)
> > @@ -953,12 +958,12 @@ static bool khugepaged_prealloc_page(struct page **hpage, bool *wait)
> >       return true;
> >  }
> >
> > -static struct page *
> > -khugepaged_alloc_page(struct page **hpage, gfp_t gfp, int node)
> > +static struct page *khugepaged_alloc_page(struct collapse_control *cc,
> > +                                       gfp_t gfp, int node)
> >  {
> > -     VM_BUG_ON(!*hpage);
> > +     VM_BUG_ON(!cc->hpage);
> >
> > -     return  *hpage;
> > +     return cc->hpage;
> >  }
> >  #endif
> >
> > @@ -1080,10 +1085,9 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> >       return true;
> >  }
> >
> > -static void collapse_huge_page(struct mm_struct *mm,
> > -                                unsigned long address,
> > -                                struct page **hpage,
> > -                                int node, int referenced, int unmapped)
> > +static void collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > +                            struct collapse_control *cc, int referenced,
> > +                            int unmapped)
> >  {
> >       LIST_HEAD(compound_pagelist);
> >       pmd_t *pmd, _pmd;
> > @@ -1096,11 +1100,12 @@ static void collapse_huge_page(struct mm_struct *mm,
> >       struct mmu_notifier_range range;
> >       gfp_t gfp;
> >       const struct cpumask *cpumask;
> > +     int node;
> >
> >       VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> >
> >       /* Only allocate from the target node */
> > -     gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
> > +     gfp = cc->gfp() | __GFP_THISNODE;
> >
> >       /*
> >        * Before allocating the hugepage, release the mmap_lock read lock.
> > @@ -1110,13 +1115,14 @@ static void collapse_huge_page(struct mm_struct *mm,
> >        */
> >       mmap_read_unlock(mm);
> >
> > +     node = khugepaged_find_target_node(cc);
> >       /* sched to specified node before huage page memory copy */
> >       if (task_node(current) != node) {
> >               cpumask = cpumask_of_node(node);
> >               if (!cpumask_empty(cpumask))
> >                       set_cpus_allowed_ptr(current, cpumask);
> >       }
> > -     new_page = khugepaged_alloc_page(hpage, gfp, node);
> > +     new_page = cc->alloc_hpage(cc, gfp, node);
>
> AFAICT you removed all references of khugepaged_alloc_page() in this patch,
> then you'd better drop the function for both NUMA and UMA.
>

Sorry, I'm not sure I follow. In khugepaged context, logic WRT
khugepaged_alloc_page() is unchanged - it's just called indirectly
through ->alloc_hpage().

> Said that, I think it's actually better to keep them, as they do things
> useful.  For example,AFAICT this ->alloc_hpage() change can leak the hpage
> alloacted for UMA already so that's why I think keeping them makes sense,
> then iiuc the BUG_ON would trigger with UMA already.
>
> I saw that you've moved khugepaged_find_target_node() into this function
> which looks definitely good, but if we could keep khugepaged_alloc_page()
> and then IMHO we could even move khugepaged_find_target_node() into that
> and drop the "node" parameter in khugepaged_alloc_page().
>

I actually had done this, until commit 4272063db38c ("mm/khugepaged:
sched to numa node when collapse huge page") which forced me to keep
"node" visible in this function.

> I'd even consider moving cc->gfp() all into it if possible, since the gfp
> seems to be always with __GFP_THISNODE anyways.
>

I would have liked to do this, but the gfp flags are referenced again
when calling mem_cgroup_charge(), so I couldn't quite get rid of them
from here.

> What do you think?
>
> >       if (!new_page) {
> >               result = SCAN_ALLOC_HUGE_PAGE_FAIL;
> >               goto out_nolock;
> > @@ -1238,15 +1244,15 @@ static void collapse_huge_page(struct mm_struct *mm,
> >       update_mmu_cache_pmd(vma, address, pmd);
> >       spin_unlock(pmd_ptl);
> >
> > -     *hpage = NULL;
> > +     cc->hpage = NULL;
> >
> >       khugepaged_pages_collapsed++;
> >       result = SCAN_SUCCEED;
> >  out_up_write:
> >       mmap_write_unlock(mm);
> >  out_nolock:
> > -     if (!IS_ERR_OR_NULL(*hpage))
> > -             mem_cgroup_uncharge(page_folio(*hpage));
> > +     if (!IS_ERR_OR_NULL(cc->hpage))
> > +             mem_cgroup_uncharge(page_folio(cc->hpage));
> >       trace_mm_collapse_huge_page(mm, isolated, result);
> >       return;
> >  }
> > @@ -1254,7 +1260,6 @@ static void collapse_huge_page(struct mm_struct *mm,
> >  static int khugepaged_scan_pmd(struct mm_struct *mm,
> >                              struct vm_area_struct *vma,
> >                              unsigned long address,
> > -                            struct page **hpage,
> >                              struct collapse_control *cc)
> >  {
> >       pmd_t *pmd;
> > @@ -1399,10 +1404,8 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> >  out_unmap:
> >       pte_unmap_unlock(pte, ptl);
> >       if (ret) {
> > -             node = khugepaged_find_target_node(cc);
> >               /* collapse_huge_page will return with the mmap_lock released */
> > -             collapse_huge_page(mm, address, hpage, node,
> > -                             referenced, unmapped);
> > +             collapse_huge_page(mm, address, cc, referenced, unmapped);
> >       }
> >  out:
> >       trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,
> > @@ -1667,8 +1670,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
> >   * @mm: process address space where collapse happens
> >   * @file: file that collapse on
> >   * @start: collapse start address
> > - * @hpage: new allocated huge page for collapse
> > - * @node: appointed node the new huge page allocate from
> > + * @cc: collapse context and scratchpad
> >   *
> >   * Basic scheme is simple, details are more complex:
> >   *  - allocate and lock a new huge page;
> > @@ -1686,8 +1688,8 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
> >   *    + unlock and free huge page;
> >   */
> >  static void collapse_file(struct mm_struct *mm,
> > -             struct file *file, pgoff_t start,
> > -             struct page **hpage, int node)
> > +                       struct file *file, pgoff_t start,
> > +                       struct collapse_control *cc)
> >  {
> >       struct address_space *mapping = file->f_mapping;
> >       gfp_t gfp;
> > @@ -1697,15 +1699,16 @@ static void collapse_file(struct mm_struct *mm,
> >       XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
> >       int nr_none = 0, result = SCAN_SUCCEED;
> >       bool is_shmem = shmem_file(file);
> > -     int nr;
> > +     int nr, node;
> >
> >       VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
> >       VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
> >
> >       /* Only allocate from the target node */
> > -     gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
> > +     gfp = cc->gfp() | __GFP_THISNODE;
> > +     node = khugepaged_find_target_node(cc);
> >
> > -     new_page = khugepaged_alloc_page(hpage, gfp, node);
> > +     new_page = cc->alloc_hpage(cc, gfp, node);
> >       if (!new_page) {
> >               result = SCAN_ALLOC_HUGE_PAGE_FAIL;
> >               goto out;
> > @@ -1998,7 +2001,7 @@ static void collapse_file(struct mm_struct *mm,
> >                * Remove pte page tables, so we can re-fault the page as huge.
> >                */
> >               retract_page_tables(mapping, start);
> > -             *hpage = NULL;
> > +             cc->hpage = NULL;
> >
> >               khugepaged_pages_collapsed++;
> >       } else {
> > @@ -2045,14 +2048,14 @@ static void collapse_file(struct mm_struct *mm,
> >       unlock_page(new_page);
> >  out:
> >       VM_BUG_ON(!list_empty(&pagelist));
> > -     if (!IS_ERR_OR_NULL(*hpage))
> > -             mem_cgroup_uncharge(page_folio(*hpage));
> > +     if (!IS_ERR_OR_NULL(cc->hpage))
> > +             mem_cgroup_uncharge(page_folio(cc->hpage));
> >       /* TODO: tracepoints */
> >  }
> >
> >  static void khugepaged_scan_file(struct mm_struct *mm,
> > -             struct file *file, pgoff_t start, struct page **hpage,
> > -             struct collapse_control *cc)
> > +                              struct file *file, pgoff_t start,
> > +                              struct collapse_control *cc)
> >  {
> >       struct page *page = NULL;
> >       struct address_space *mapping = file->f_mapping;
> > @@ -2125,8 +2128,7 @@ static void khugepaged_scan_file(struct mm_struct *mm,
> >                       result = SCAN_EXCEED_NONE_PTE;
> >                       count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> >               } else {
> > -                     node = khugepaged_find_target_node(cc);
> > -                     collapse_file(mm, file, start, hpage, node);
> > +                     collapse_file(mm, file, start, cc);
> >               }
> >       }
> >
> > @@ -2134,8 +2136,8 @@ static void khugepaged_scan_file(struct mm_struct *mm,
> >  }
> >  #else
> >  static void khugepaged_scan_file(struct mm_struct *mm,
> > -             struct file *file, pgoff_t start, struct page **hpage,
> > -             struct collapse_control *cc)
> > +                              struct file *file, pgoff_t start,
> > +                              struct collapse_control *cc)
> >  {
> >       BUILD_BUG();
> >  }
> > @@ -2146,7 +2148,6 @@ static void khugepaged_collapse_pte_mapped_thps(struct mm_slot *mm_slot)
> >  #endif
> >
> >  static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
> > -                                         struct page **hpage,
> >                                           struct collapse_control *cc)
> >       __releases(&khugepaged_mm_lock)
> >       __acquires(&khugepaged_mm_lock)
> > @@ -2223,12 +2224,11 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
> >
> >                               mmap_read_unlock(mm);
> >                               ret = 1;
> > -                             khugepaged_scan_file(mm, file, pgoff, hpage, cc);
> > +                             khugepaged_scan_file(mm, file, pgoff, cc);
> >                               fput(file);
> >                       } else {
> >                               ret = khugepaged_scan_pmd(mm, vma,
> > -                                             khugepaged_scan.address,
> > -                                             hpage, cc);
> > +                                             khugepaged_scan.address, cc);
> >                       }
> >                       /* move to next address */
> >                       khugepaged_scan.address += HPAGE_PMD_SIZE;
> > @@ -2286,15 +2286,15 @@ static int khugepaged_wait_event(void)
> >
> >  static void khugepaged_do_scan(struct collapse_control *cc)
> >  {
> > -     struct page *hpage = NULL;
> >       unsigned int progress = 0, pass_through_head = 0;
> >       unsigned int pages = READ_ONCE(khugepaged_pages_to_scan);
> >       bool wait = true;
> >
> > +     cc->hpage = NULL;
> >       lru_add_drain_all();
> >
> >       while (progress < pages) {
> > -             if (!khugepaged_prealloc_page(&hpage, &wait))
> > +             if (!khugepaged_prealloc_page(&cc->hpage, &wait))
> >                       break;
> >
> >               cond_resched();
> > @@ -2308,14 +2308,14 @@ static void khugepaged_do_scan(struct collapse_control *cc)
> >               if (khugepaged_has_work() &&
> >                   pass_through_head < 2)
> >                       progress += khugepaged_scan_mm_slot(pages - progress,
> > -                                                         &hpage, cc);
> > +                                                         cc);
> >               else
> >                       progress = pages;
> >               spin_unlock(&khugepaged_mm_lock);
> >       }
> >
> > -     if (!IS_ERR_OR_NULL(hpage))
> > -             put_page(hpage);
> > +     if (!IS_ERR_OR_NULL(cc->hpage))
> > +             put_page(cc->hpage);
> >  }
> >
> >  static bool khugepaged_should_wakeup(void)
> > @@ -2349,6 +2349,8 @@ static int khugepaged(void *none)
> >       struct mm_slot *mm_slot;
> >       struct collapse_control cc = {
> >               .last_target_node = NUMA_NO_NODE,
> > +             .gfp = &alloc_hugepage_khugepaged_gfpmask,
> > +             .alloc_hpage = &khugepaged_alloc_page,
>
> I'm also wondering whether we could avoid the gfp() hook, because logically
> that can be inlined into the ->alloc_hpage() hook, then we don't need gfp
> parameter anymore for ->alloc_hpage(), which seems to be a win-win?
>

As mentioned above, I would have liked to not have this, but the gfp
flags are used in a couple places (mem_cgroup_charge() and
->alloc_hpage()).

I could (?) make .gfp of type gfp_t and just update it on every
khugepaged scan (in case it changed) and also remove the gfp parameter
for ->alloc_hpage().

Thank you, Peter!

Zach

> --
> Peter Xu
>