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Tue, 31 Mar 2026 14:09:30 -0700 (PDT) X-Received: by 2002:a53:ea4d:0:b0:64e:e90c:5682 with SMTP id 956f58d0204a3-6502feb45a1mr754973d50.66.1774991369882; Tue, 31 Mar 2026 14:09:29 -0700 (PDT) MIME-Version: 1.0 References: <20260325114022.444081-1-npache@redhat.com> <20260325114022.444081-6-npache@redhat.com> <7760c811-e100-4d40-9217-0813c28314be@lucifer.local> <0223d45b-e8b4-49a7-882b-477250d0c14d@kernel.org> <26fcec86-ee78-4f5c-8ca5-0e6230699cfd@kernel.org> <440d6b8a-d299-43eb-83b5-6625f9468e5e@lucifer.local> <37132e09-9c03-4afc-bc73-4bca416c5343@kernel.org> <9d5a4703-a15e-4be6-926b-af2d98db40c0@kernel.org> In-Reply-To: <9d5a4703-a15e-4be6-926b-af2d98db40c0@kernel.org> From: Nico Pache Date: Tue, 31 Mar 2026 15:09:08 -0600 X-Gm-Features: AQROBzCrJ0dOKq7MHwKLsq9qklkbNmoSJ-7ELrRDpcJn1e5iBPnmBJmPC1e3vBk Message-ID: Subject: Re: [PATCH mm-unstable v4 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd() To: "David Hildenbrand (Arm)" Cc: "Lorenzo Stoakes (Oracle)" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jackmanb@google.com, jack@suse.cz, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: zZjtjMIRbIPNfQ-ib-J7GWhJbBpx4-ty7ghlDXsKwZM_1774991370 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 26D40C0003 X-Stat-Signature: fg6pk1km3tnrhy965hae9s1taisqrn8y X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1774991373-402157 X-HE-Meta: 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 IgvIUWkO VIf/m95ZbOLOrqmdKf8dRjexkKKYeEbe8R/uLDDZbk81K/y0bMgkhYc0XNU+B/J+/EhLrsK7FryWE+WCprZVKBPapiujXrfMvFpXaas5QpJwWhqLQyKXA5WHcI9DKDDjJMQSo Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Mar 31, 2026 at 3:04=E2=80=AFPM David Hildenbrand (Arm) wrote: > > On 3/31/26 22:50, David Hildenbrand (Arm) wrote: > > On 3/31/26 22:06, Lorenzo Stoakes (Oracle) wrote: > >> On Tue, Mar 31, 2026 at 10:00:58PM +0200, David Hildenbrand (Arm) wrot= e: > >>> > >>> Right. > >>> > >>> The original code used > >>> > >>> bool mmap_locked =3D true; > >>> > >>> If the fix get squashed, maybe we should revert to that handling to > >>> minimize the churn? > >> > >> Well collapse_single_pmd() and all descendents would need to be update= d to > >> invert the boolean, not sure it's worth it. > > > > The code is confusing me. > > > > In the old code, collapse_single_pmd() was called with "mmap_locked =3D= =3D > > true" and set "mmap_locked=3Dfalse". > > > > Now we call it with "mmap_unlocked=3Dfalse" and it sets "mmap_unlocked= =3Dtrue". > > > > So far so good. > > > > However, collapse_scan_pmd() consumed "mmap_locked", now it effectively > > consumes "mmap_unlocked". > > Okay, I'm too confused for today and give up :) > > FWIW, this is the effective change, and the effective changes to > mmap_unlocked and lock_dropped ... confuse me: basically all the logic was inverted. > > > mm/khugepaged.c | 146 +++++++++++++++++++++++++----------------------- > 1 file changed, 76 insertions(+), 70 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 26c38934e829..3ffff9f98b2f 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1253,7 +1253,7 @@ static enum scan_result collapse_huge_page(struct m= m_struct *mm, unsigned long a > > static enum scan_result collapse_scan_pmd(struct mm_struct *mm, > struct vm_area_struct *vma, unsigned long start_addr, > - bool *mmap_locked, struct collapse_control *cc) > + bool *lock_dropped, struct collapse_control *cc) > { > pmd_t *pmd; > pte_t *pte, *_pte; > @@ -1428,7 +1428,7 @@ static enum scan_result collapse_scan_pmd(struct mm= _struct *mm, > result =3D collapse_huge_page(mm, start_addr, referenced, > unmapped, cc); > /* collapse_huge_page will return with the mmap_lock rele= ased */ > - *mmap_locked =3D false; > + *lock_dropped =3D true; > } > out: > trace_mm_khugepaged_scan_pmd(mm, folio, referenced, > @@ -2420,6 +2420,67 @@ static enum scan_result collapse_scan_file(struct = mm_struct *mm, > return result; > } > > +/* > + * Try to collapse a single PMD starting at a PMD aligned addr, and retu= rn > + * the results. > + */ > +static enum scan_result collapse_single_pmd(unsigned long addr, > + struct vm_area_struct *vma, bool *lock_dropped, > + struct collapse_control *cc) > +{ > + struct mm_struct *mm =3D vma->vm_mm; > + bool triggered_wb =3D false; > + enum scan_result result; > + struct file *file; > + pgoff_t pgoff; > + > + mmap_assert_locked(mm); > + > + if (vma_is_anonymous(vma)) { > + result =3D collapse_scan_pmd(mm, vma, addr, lock_dropped,= cc); > + goto end; > + } > + > + file =3D get_file(vma->vm_file); > + pgoff =3D linear_page_index(vma, addr); > + > + mmap_read_unlock(mm); > + *lock_dropped =3D true; > +retry: > + result =3D collapse_scan_file(mm, addr, file, pgoff, cc); > + > + /* > + * For MADV_COLLAPSE, when encountering dirty pages, try to write= back, > + * then retry the collapse one time. > + */ > + if (!cc->is_khugepaged && result =3D=3D SCAN_PAGE_DIRTY_OR_WRITEB= ACK && > + !triggered_wb && mapping_can_writeback(file->f_mapping)) { > + const loff_t lstart =3D (loff_t)pgoff << PAGE_SHIFT; > + const loff_t lend =3D lstart + HPAGE_PMD_SIZE - 1; > + > + filemap_write_and_wait_range(file->f_mapping, lstart, len= d); > + triggered_wb =3D true; > + goto retry; > + } > + fput(file); > + > + if (result =3D=3D SCAN_PTE_MAPPED_HUGEPAGE) { > + mmap_read_lock(mm); > + if (collapse_test_exit_or_disable(mm)) > + result =3D SCAN_ANY_PROCESS; > + else > + result =3D try_collapse_pte_mapped_thp(mm, addr, > + !cc->is_khug= epaged); > + if (result =3D=3D SCAN_PMD_MAPPED) > + result =3D SCAN_SUCCEED; > + mmap_read_unlock(mm); > + } > +end: > + if (cc->is_khugepaged && result =3D=3D SCAN_SUCCEED) > + ++khugepaged_pages_collapsed; > + return result; > +} > + > static void collapse_scan_mm_slot(unsigned int progress_max, > enum scan_result *result, struct collapse_control *cc) > __releases(&khugepaged_mm_lock) > @@ -2481,46 +2542,21 @@ static void collapse_scan_mm_slot(unsigned int pr= ogress_max, > VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK); > > while (khugepaged_scan.address < hend) { > - bool mmap_locked =3D true; > + bool lock_dropped =3D false; > > cond_resched(); > if (unlikely(collapse_test_exit_or_disable(mm))) > goto breakouterloop; > > - VM_BUG_ON(khugepaged_scan.address < hstart || > + VM_WARN_ON_ONCE(khugepaged_scan.address < hstart = || > khugepaged_scan.address + HPAGE_PMD_SIZ= E > > hend); > - if (!vma_is_anonymous(vma)) { > - struct file *file =3D get_file(vma->vm_fi= le); > - pgoff_t pgoff =3D linear_page_index(vma, > - khugepaged_scan.address); > - > - mmap_read_unlock(mm); > - mmap_locked =3D false; > - *result =3D collapse_scan_file(mm, > - khugepaged_scan.address, file, pg= off, cc); > - fput(file); > - if (*result =3D=3D SCAN_PTE_MAPPED_HUGEPA= GE) { > - mmap_read_lock(mm); > - if (collapse_test_exit_or_disable= (mm)) > - goto breakouterloop; > - *result =3D try_collapse_pte_mapp= ed_thp(mm, > - khugepaged_scan.address, = false); > - if (*result =3D=3D SCAN_PMD_MAPPE= D) > - *result =3D SCAN_SUCCEED; > - mmap_read_unlock(mm); > - } > - } else { > - *result =3D collapse_scan_pmd(mm, vma, > - khugepaged_scan.address, &mmap_lo= cked, cc); > - } > - > - if (*result =3D=3D SCAN_SUCCEED) > - ++khugepaged_pages_collapsed; > > + *result =3D collapse_single_pmd(khugepaged_scan.a= ddress, > + vma, &lock_dropped,= cc); > /* move to next address */ > khugepaged_scan.address +=3D HPAGE_PMD_SIZE; > - if (!mmap_locked) > + if (lock_dropped) > /* > * We released mmap_lock so break loop. = Note > * that we drop mmap_lock before all huge= page > @@ -2795,7 +2831,7 @@ int madvise_collapse(struct vm_area_struct *vma, un= signed long start, > unsigned long hstart, hend, addr; > enum scan_result last_fail =3D SCAN_FAIL; > int thps =3D 0; > - bool mmap_locked =3D true; > + bool mmap_unlocked =3D false; > > BUG_ON(vma->vm_start > start); > BUG_ON(vma->vm_end < end); > @@ -2817,13 +2853,12 @@ int madvise_collapse(struct vm_area_struct *vma, = unsigned long start, > > for (addr =3D hstart; addr < hend; addr +=3D HPAGE_PMD_SIZE) { > enum scan_result result =3D SCAN_FAIL; > - bool triggered_wb =3D false; > > -retry: > - if (!mmap_locked) { > + if (mmap_unlocked) { > cond_resched(); > mmap_read_lock(mm); > - mmap_locked =3D true; > + mmap_unlocked =3D false; > + *lock_dropped =3D true; > result =3D hugepage_vma_revalidate(mm, addr, fals= e, &vma, > cc); > if (result !=3D SCAN_SUCCEED) { > @@ -2833,45 +2868,14 @@ int madvise_collapse(struct vm_area_struct *vma, = unsigned long start, > > hend =3D min(hend, vma->vm_end & HPAGE_PMD_MASK); > } > - mmap_assert_locked(mm); > - if (!vma_is_anonymous(vma)) { > - struct file *file =3D get_file(vma->vm_file); > - pgoff_t pgoff =3D linear_page_index(vma, addr); > > - mmap_read_unlock(mm); > - mmap_locked =3D false; > - *lock_dropped =3D true; > - result =3D collapse_scan_file(mm, addr, file, pgo= ff, cc); > - > - if (result =3D=3D SCAN_PAGE_DIRTY_OR_WRITEBACK &&= !triggered_wb && > - mapping_can_writeback(file->f_mapping)) { > - loff_t lstart =3D (loff_t)pgoff << PAGE_S= HIFT; > - loff_t lend =3D lstart + HPAGE_PMD_SIZE -= 1; > + result =3D collapse_single_pmd(addr, vma, &mmap_unlocked,= cc); > > - filemap_write_and_wait_range(file->f_mapp= ing, lstart, lend); > - triggered_wb =3D true; > - fput(file); > - goto retry; > - } > - fput(file); > - } else { > - result =3D collapse_scan_pmd(mm, vma, addr, &mmap= _locked, cc); > - } > - if (!mmap_locked) > - *lock_dropped =3D true; > - > -handle_result: > switch (result) { > case SCAN_SUCCEED: > case SCAN_PMD_MAPPED: > ++thps; > break; > - case SCAN_PTE_MAPPED_HUGEPAGE: > - BUG_ON(mmap_locked); > - mmap_read_lock(mm); > - result =3D try_collapse_pte_mapped_thp(mm, addr, = true); > - mmap_read_unlock(mm); > - goto handle_result; > /* Whitelisted set of results where continuing OK */ > case SCAN_NO_PTE_TABLE: > case SCAN_PTE_NON_PRESENT: > @@ -2894,8 +2898,10 @@ int madvise_collapse(struct vm_area_struct *vma, u= nsigned long start, > > out_maybelock: > /* Caller expects us to hold mmap_lock on return */ > - if (!mmap_locked) > + if (mmap_unlocked) { > + *lock_dropped =3D false; Lorenzo, is this correct? At first glance, shouldn't this be `locked_dropped =3D true`? -- Nico > mmap_read_lock(mm); > + } > out_nolock: > mmap_assert_locked(mm); > mmdrop(mm); > -- > 2.43.0 > > > -- > Cheers, > > David >