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Message-ID: <CAA1CXcA1btm3ddHzKfrCZcce8=ekVKfbo84=4SegabQSgqoeWw@mail.gmail.com>
Date: Tue, 18 Feb 2025 15:24:49 -0700
From: Nico Pache <npache@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
linux-mm@...ck.org, anshuman.khandual@....com, catalin.marinas@....com,
cl@...two.org, vbabka@...e.cz, mhocko@...e.com, apopple@...dia.com,
dave.hansen@...ux.intel.com, will@...nel.org, baohua@...nel.org, jack@...e.cz,
srivatsa@...il.mit.edu, haowenchao22@...il.com, hughd@...gle.com,
aneesh.kumar@...nel.org, yang@...amperecomputing.com, peterx@...hat.com,
ioworker0@...il.com, wangkefeng.wang@...wei.com, ziy@...dia.com,
jglisse@...gle.com, surenb@...gle.com, vishal.moola@...il.com,
zokeefe@...gle.com, zhengqi.arch@...edance.com, jhubbard@...dia.com,
21cnbao@...il.com, willy@...radead.org, kirill.shutemov@...ux.intel.com,
david@...hat.com, aarcange@...hat.com, raquini@...hat.com, dev.jain@....com,
sunnanyong@...wei.com, usamaarif642@...il.com, audra@...hat.com,
akpm@...ux-foundation.org, rostedt@...dmis.org,
mathieu.desnoyers@...icios.com, tiwai@...e.de
Subject: Re: [RFC v2 1/9] introduce khugepaged_collapse_single_pmd to unify
khugepaged and madvise_collapse
On Tue, Feb 18, 2025 at 9:26 AM Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 11/02/2025 00:30, Nico Pache wrote:
> > The khugepaged daemon and madvise_collapse have two different
> > implementations that do almost the same thing.
> >
> > Create khugepaged_collapse_single_pmd to increase code
> > reuse and create an entry point for future khugepaged changes.
> >
> > Refactor madvise_collapse and khugepaged_scan_mm_slot to use
> > the new khugepaged_collapse_single_pmd function.
> >
> > Signed-off-by: Nico Pache <npache@...hat.com>
> > ---
> > mm/khugepaged.c | 96 +++++++++++++++++++++++++------------------------
> > 1 file changed, 50 insertions(+), 46 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 5f0be134141e..46faee67378b 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2365,6 +2365,52 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
> > }
> > #endif
> >
> > +/*
> > + * Try to collapse a single PMD starting at a PMD aligned addr, and return
> > + * the results.
> > + */
> > +static int khugepaged_collapse_single_pmd(unsigned long addr, struct mm_struct *mm,
> > + struct vm_area_struct *vma, bool *mmap_locked,
> > + struct collapse_control *cc)
>
> nit: given the vma links to the mm is it really neccessary to pass both? Why not
> just pass vma?
Ah good point!
>
> > +{
> > + int result = SCAN_FAIL;
> > + unsigned long tva_flags = cc->is_khugepaged ? TVA_ENFORCE_SYSFS : 0;
> > +
> > + if (!*mmap_locked) {
> > + mmap_read_lock(mm);
> > + *mmap_locked = true;
> > + }
>
> AFAICT, the read lock is always held when khugepaged_collapse_single_pmd() is
> called. Perhaps VM_WARN_ON(!*mmap_locked) would be more appropriate?
Hmm, I'm actually not sure why that's there. Its probably left over
from a previous change I was trying a while back.
>
> > +
> > + if (thp_vma_allowable_order(vma, vma->vm_flags,
> > + tva_flags, PMD_ORDER)) {
> > + if (IS_ENABLED(CONFIG_SHMEM) && !vma_is_anonymous(vma)) {
>
> I guess it was like this before, but what's the relevance of CONFIG_SHMEM?
> Surely this should work for any file if CONFIG_READ_ONLY_THP_FOR_FS is enabled?
Yeah I think David brought up a similar point during one of our
meetings. I'm just combining the collapse users, so I'll leave it for
now.
>
> > + struct file *file = get_file(vma->vm_file);
> > + pgoff_t pgoff = linear_page_index(vma, addr);
> > +
> > + mmap_read_unlock(mm);
> > + *mmap_locked = false;
> > + result = hpage_collapse_scan_file(mm, addr, file, pgoff,
> > + cc);
> > + fput(file);
> > + if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > + mmap_read_lock(mm);
> > + if (hpage_collapse_test_exit_or_disable(mm))
> > + goto end;
> > + result = collapse_pte_mapped_thp(mm, addr,
> > + !cc->is_khugepaged);
> > + mmap_read_unlock(mm);
> > + }
> > + } else {
> > + result = hpage_collapse_scan_pmd(mm, vma, addr,
> > + mmap_locked, cc);
> > + }
> > + if (result == SCAN_SUCCEED || result == SCAN_PMD_MAPPED)
> > + ++khugepaged_pages_collapsed;
>
> Looks like this counter was previously only incremented for the scan path, not
> for the madvise_collapse path. Not sure if that's a problem?
Yep! Usama noted that too, already fixed in the next version :)
Thanks!
-- Nico
>
> Thanks,
> Ryan
>
> > + }
> > +end:
> > + return result;
> > +}
> > +
> > static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> > struct collapse_control *cc)
> > __releases(&khugepaged_mm_lock)
> > @@ -2439,33 +2485,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> > VM_BUG_ON(khugepaged_scan.address < hstart ||
> > khugepaged_scan.address + HPAGE_PMD_SIZE >
> > hend);
> > - if (IS_ENABLED(CONFIG_SHMEM) && !vma_is_anonymous(vma)) {
> > - struct file *file = get_file(vma->vm_file);
> > - pgoff_t pgoff = linear_page_index(vma,
> > - khugepaged_scan.address);
> >
> > - mmap_read_unlock(mm);
> > - mmap_locked = false;
> > - *result = hpage_collapse_scan_file(mm,
> > - khugepaged_scan.address, file, pgoff, cc);
> > - fput(file);
> > - if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > - mmap_read_lock(mm);
> > - if (hpage_collapse_test_exit_or_disable(mm))
> > - goto breakouterloop;
> > - *result = collapse_pte_mapped_thp(mm,
> > - khugepaged_scan.address, false);
> > - if (*result == SCAN_PMD_MAPPED)
> > - *result = SCAN_SUCCEED;
> > - mmap_read_unlock(mm);
> > - }
> > - } else {
> > - *result = hpage_collapse_scan_pmd(mm, vma,
> > - khugepaged_scan.address, &mmap_locked, cc);
> > - }
> > -
> > - if (*result == SCAN_SUCCEED)
> > - ++khugepaged_pages_collapsed;
> > + *result = khugepaged_collapse_single_pmd(khugepaged_scan.address,
> > + mm, vma, &mmap_locked, cc);
> >
> > /* move to next address */
> > khugepaged_scan.address += HPAGE_PMD_SIZE;
> > @@ -2785,36 +2807,18 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
> > mmap_assert_locked(mm);
> > memset(cc->node_load, 0, sizeof(cc->node_load));
> > nodes_clear(cc->alloc_nmask);
> > - if (IS_ENABLED(CONFIG_SHMEM) && !vma_is_anonymous(vma)) {
> > - struct file *file = get_file(vma->vm_file);
> > - pgoff_t pgoff = linear_page_index(vma, addr);
> >
> > - mmap_read_unlock(mm);
> > - mmap_locked = false;
> > - result = hpage_collapse_scan_file(mm, addr, file, pgoff,
> > - cc);
> > - fput(file);
> > - } else {
> > - result = hpage_collapse_scan_pmd(mm, vma, addr,
> > - &mmap_locked, cc);
> > - }
> > + result = khugepaged_collapse_single_pmd(addr, mm, vma, &mmap_locked, cc);
> > +
> > if (!mmap_locked)
> > *prev = NULL; /* Tell caller we dropped mmap_lock */
> >
> > -handle_result:
> > switch (result) {
> > case SCAN_SUCCEED:
> > case SCAN_PMD_MAPPED:
> > ++thps;
> > break;
> > case SCAN_PTE_MAPPED_HUGEPAGE:
> > - BUG_ON(mmap_locked);
> > - BUG_ON(*prev);
> > - mmap_read_lock(mm);
> > - result = collapse_pte_mapped_thp(mm, addr, true);
> > - mmap_read_unlock(mm);
> > - goto handle_result;
> > - /* Whitelisted set of results where continuing OK */
> > case SCAN_PMD_NULL:
> > case SCAN_PTE_NON_PRESENT:
> > case SCAN_PTE_UFFD_WP:
>
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