Thanks for taking the time to review, Peter!
On Tue, Apr 26, 2022 at 5:26 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> Hi, Zach,
>
> On Tue, Apr 26, 2022 at 07:44:01AM -0700, Zach O'Keefe wrote:
> > When scanning an anon pmd to see if it's eligible for collapse, return
> > SCAN_PMD_MAPPED if the pmd already maps a THP. Note that
> > SCAN_PMD_MAPPED is different from SCAN_PAGE_COMPOUND used in the
> > file-collapse path, since the latter might identify pte-mapped compound
> > pages. This is required by MADV_COLLAPSE which necessarily needs to
> > know what hugepage-aligned/sized regions are already pmd-mapped.
> >
> > Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
>
> IIUC we don't need to attach this reported-by if this is not a bugfix. I
> think you can simply fix all issues reported by the test bot and only
> attach the line if the patch is fixing the problem that the bot was
> reporting explicitly.
>
Ya, I wasn't entirely sure what to do here, but including seems to not
be without precedent, e.g. commit 92bbef67d459 ("mm: make
alloc_contig_range work at pageblock granularity"), and likewise just
wanted to give credit where I thought it was due. Though, I suppose
folks who catch bugs in the review process aren't ack'd similarly, so
perhaps it does make sense to remove this.
> > ---
> > include/trace/events/huge_memory.h | 3 ++-
> > mm/internal.h | 1 +
> > mm/khugepaged.c | 30 ++++++++++++++++++++++++++----
> > mm/rmap.c | 15 +++++++++++++--
> > 4 files changed, 42 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> > index d651f3437367..9faa678e0a5b 100644
> > --- a/include/trace/events/huge_memory.h
> > +++ b/include/trace/events/huge_memory.h
> > @@ -33,7 +33,8 @@
> > EM( SCAN_ALLOC_HUGE_PAGE_FAIL, "alloc_huge_page_failed") \
> > EM( SCAN_CGROUP_CHARGE_FAIL, "ccgroup_charge_failed") \
> > EM( SCAN_TRUNCATED, "truncated") \
> > - EMe(SCAN_PAGE_HAS_PRIVATE, "page_has_private") \
> > + EM( SCAN_PAGE_HAS_PRIVATE, "page_has_private") \
> > + EMe(SCAN_PMD_MAPPED, "page_pmd_mapped") \
>
> Nit: IMHO it can be put even in the middle so we don't need to touch the
> EMe() every time. :)
>
> Apart from that, it does sound proper to me to put SCAN_PMD_MAPPED to be
> right after SCAN_PMD_NULL anyway.
>
Makes sense to me. Done.
> >
> > #undef EM
> > #undef EMe
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 0667abd57634..51ae9f71a2a3 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -172,6 +172,7 @@ extern void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason
> > /*
> > * in mm/rmap.c:
> > */
> > +pmd_t *mm_find_pmd_raw(struct mm_struct *mm, unsigned long address);
> > extern pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
> >
> > /*
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index ba8dbd1825da..2933b13fc975 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -51,6 +51,7 @@ enum scan_result {
> > SCAN_CGROUP_CHARGE_FAIL,
> > SCAN_TRUNCATED,
> > SCAN_PAGE_HAS_PRIVATE,
> > + SCAN_PMD_MAPPED,
> > };
> >
> > #define CREATE_TRACE_POINTS
> > @@ -987,6 +988,29 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> > return 0;
> > }
> >
> > +static int find_pmd_or_thp_or_none(struct mm_struct *mm,
> > + unsigned long address,
> > + pmd_t **pmd)
> > +{
> > + pmd_t pmde;
> > +
> > + *pmd = mm_find_pmd_raw(mm, address);
> > + if (!*pmd)
> > + return SCAN_PMD_NULL;
> > +
> > + pmde = pmd_read_atomic(*pmd);
> > +
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > + /* See comments in pmd_none_or_trans_huge_or_clear_bad() */
> > + barrier();
> > +#endif
> > + if (!pmd_present(pmde) || pmd_none(pmde))
>
> Could we drop the pmd_none() check? I assume !pmd_present() should have
> covered that case already?
>
I opted for safety here since I didn't know if pmd_present() always
implied !pmd_none() on all archs, but given mm_find_pmd() elides the
check, perhaps it's safe to do so here. Thanks for the suggestion.
> > + return SCAN_PMD_NULL;
> > + if (pmd_trans_huge(pmde))
> > + return SCAN_PMD_MAPPED;
> > + return SCAN_SUCCEED;
> > +}
> > +
> > /*
> > * Bring missing pages in from swap, to complete THP collapse.
> > * Only done if khugepaged_scan_pmd believes it is worthwhile.
> > @@ -1238,11 +1262,9 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> >
> > VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> >
> > - pmd = mm_find_pmd(mm, address);
> > - if (!pmd) {
> > - result = SCAN_PMD_NULL;
> > + result = find_pmd_or_thp_or_none(mm, address, &pmd);
> > + if (result != SCAN_SUCCEED)
> > goto out;
> > - }
> >
> > memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
> > pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 61e63db5dc6f..49817f35e65c 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -759,13 +759,12 @@ unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
> > return vma_address(page, vma);
> > }
> >
> > -pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
> > +pmd_t *mm_find_pmd_raw(struct mm_struct *mm, unsigned long address)
> > {
> > pgd_t *pgd;
> > p4d_t *p4d;
> > pud_t *pud;
> > pmd_t *pmd = NULL;
> > - pmd_t pmde;
> >
> > pgd = pgd_offset(mm, address);
> > if (!pgd_present(*pgd))
> > @@ -780,6 +779,18 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
> > goto out;
> >
> > pmd = pmd_offset(pud, address);
> > +out:
> > + return pmd;
> > +}
> > +
> > +pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
> > +{
> > + pmd_t pmde;
> > + pmd_t *pmd;
> > +
> > + pmd = mm_find_pmd_raw(mm, address);
> > + if (!pmd)
> > + goto out;
> > /*
> > * Some THP functions use the sequence pmdp_huge_clear_flush(), set_pmd_at()
> > * without holding anon_vma lock for write. So when looking for a
> > --
> > 2.36.0.rc2.479.g8af0fa9b8e-goog
> >
>
> --
> Peter Xu
>
