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[109.81.89.112]) by smtp.gmail.com with UTF8SMTPSA id 4fb4d7f45d1cf-60566c5a758sm9856845a12.19.2025.06.05.00.10.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Jun 2025 00:10:55 -0700 (PDT) Date: Thu, 5 Jun 2025 09:10:55 +0200 From: Michal Hocko To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Jason Gunthorpe , John Hubbard , Peter Xu Subject: Re: [PATCH v1] mm/gup: remove (VM_)BUG_ONs Message-ID: References: <20250604140544.688711-1-david@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250604140544.688711-1-david@redhat.com> On Wed 04-06-25 16:05:44, David Hildenbrand wrote: > Especially once we hit one of the assertions in > sanity_check_pinned_pages(), observing follow-up assertions failing > in other code can give good clues about what went wrong, so use > VM_WARN_ON_ONCE instead. > > While at it, let's just convert all VM_BUG_ON to VM_WARN_ON_ONCE as > well. Add one comment for the pfn_valid() check. > > We have to introduce VM_WARN_ON_ONCE_VMA() to make that fly. > > Drop the BUG_ON after mmap_read_lock_killable(), if that ever returns > something > 0 we're in bigger trouble. Convert the other BUG_ON's into > VM_WARN_ON_ONCE as well, they are in a similar domain "should never > happen", but more reasonable to check for during early testing. The patch itself makes sense and I think it is good time to revisit old discussion [1] and finally drop VM_BUG_ON altogether and replace it by VM_WARN_ON which could be still a useful debugging aid. [1] https://lore.kernel.org/all/c9abf109-80f2-88f5-4aae-d6fd4a30bcd3@google.com/T/#u > > Cc: Andrew Morton > Cc: Lorenzo Stoakes > Cc: "Liam R. Howlett" > Cc: Vlastimil Babka > Cc: Mike Rapoport > Cc: Suren Baghdasaryan > Cc: Michal Hocko > Cc: Jason Gunthorpe > Cc: John Hubbard > Cc: Peter Xu > Signed-off-by: David Hildenbrand Acked-by: Michal Hocko > --- > > Wanted to do this for a long time, but my todo list keeps growing ... > > Based on mm/mm-unstable > > --- > include/linux/mmdebug.h | 12 ++++++++++++ > mm/gup.c | 41 +++++++++++++++++++---------------------- > 2 files changed, 31 insertions(+), 22 deletions(-) > > diff --git a/include/linux/mmdebug.h b/include/linux/mmdebug.h > index a0a3894900ed4..14a45979cccc9 100644 > --- a/include/linux/mmdebug.h > +++ b/include/linux/mmdebug.h > @@ -89,6 +89,17 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi); > } \ > unlikely(__ret_warn_once); \ > }) > +#define VM_WARN_ON_ONCE_VMA(cond, vma) ({ \ > + static bool __section(".data..once") __warned; \ > + int __ret_warn_once = !!(cond); \ > + \ > + if (unlikely(__ret_warn_once && !__warned)) { \ > + dump_vma(vma); \ > + __warned = true; \ > + WARN_ON(1); \ > + } \ > + unlikely(__ret_warn_once); \ > +}) > #define VM_WARN_ON_VMG(cond, vmg) ({ \ > int __ret_warn = !!(cond); \ > \ > @@ -115,6 +126,7 @@ void vma_iter_dump_tree(const struct vma_iterator *vmi); > #define VM_WARN_ON_FOLIO(cond, folio) BUILD_BUG_ON_INVALID(cond) > #define VM_WARN_ON_ONCE_FOLIO(cond, folio) BUILD_BUG_ON_INVALID(cond) > #define VM_WARN_ON_ONCE_MM(cond, mm) BUILD_BUG_ON_INVALID(cond) > +#define VM_WARN_ON_ONCE_VMA(cond, vma) BUILD_BUG_ON_INVALID(cond) > #define VM_WARN_ON_VMG(cond, vmg) BUILD_BUG_ON_INVALID(cond) > #define VM_WARN_ONCE(cond, format...) BUILD_BUG_ON_INVALID(cond) > #define VM_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond) > diff --git a/mm/gup.c b/mm/gup.c > index e065a49842a87..3c3931fcdd820 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -64,11 +64,11 @@ static inline void sanity_check_pinned_pages(struct page **pages, > !folio_test_anon(folio)) > continue; > if (!folio_test_large(folio) || folio_test_hugetlb(folio)) > - VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page); > + VM_WARN_ON_ONCE_PAGE(!PageAnonExclusive(&folio->page), page); > else > /* Either a PTE-mapped or a PMD-mapped THP. */ > - VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page) && > - !PageAnonExclusive(page), page); > + VM_WARN_ON_ONCE_PAGE(!PageAnonExclusive(&folio->page) && > + !PageAnonExclusive(page), page); > } > } > > @@ -760,8 +760,8 @@ static struct page *follow_huge_pmd(struct vm_area_struct *vma, > if (!pmd_write(pmdval) && gup_must_unshare(vma, flags, page)) > return ERR_PTR(-EMLINK); > > - VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) && > - !PageAnonExclusive(page), page); > + VM_WARN_ON_ONCE_PAGE((flags & FOLL_PIN) && PageAnon(page) && > + !PageAnonExclusive(page), page); > > ret = try_grab_folio(page_folio(page), 1, flags); > if (ret) > @@ -899,8 +899,8 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, > goto out; > } > > - VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) && > - !PageAnonExclusive(page), page); > + VM_WARN_ON_ONCE_PAGE((flags & FOLL_PIN) && PageAnon(page) && > + !PageAnonExclusive(page), page); > > /* try_grab_folio() does nothing unless FOLL_GET or FOLL_PIN is set. */ > ret = try_grab_folio(folio, 1, flags); > @@ -1180,7 +1180,7 @@ static int faultin_page(struct vm_area_struct *vma, > if (unshare) { > fault_flags |= FAULT_FLAG_UNSHARE; > /* FAULT_FLAG_WRITE and FAULT_FLAG_UNSHARE are incompatible */ > - VM_BUG_ON(fault_flags & FAULT_FLAG_WRITE); > + VM_WARN_ON_ONCE(fault_flags & FAULT_FLAG_WRITE); > } > > ret = handle_mm_fault(vma, address, fault_flags, NULL); > @@ -1760,10 +1760,7 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm, > } > > /* VM_FAULT_RETRY or VM_FAULT_COMPLETED cannot return errors */ > - if (!*locked) { > - BUG_ON(ret < 0); > - BUG_ON(ret >= nr_pages); > - } > + VM_WARN_ON_ONCE(!*locked && (ret < 0 || ret >= nr_pages)); > > if (ret > 0) { > nr_pages -= ret; > @@ -1808,7 +1805,6 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm, > > ret = mmap_read_lock_killable(mm); > if (ret) { > - BUG_ON(ret > 0); > if (!pages_done) > pages_done = ret; > break; > @@ -1819,11 +1815,11 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm, > pages, locked); > if (!*locked) { > /* Continue to retry until we succeeded */ > - BUG_ON(ret != 0); > + VM_WARN_ON_ONCE(ret != 0); > goto retry; > } > if (ret != 1) { > - BUG_ON(ret > 1); > + VM_WARN_ON_ONCE(ret > 1); > if (!pages_done) > pages_done = ret; > break; > @@ -1885,10 +1881,10 @@ long populate_vma_page_range(struct vm_area_struct *vma, > int gup_flags; > long ret; > > - VM_BUG_ON(!PAGE_ALIGNED(start)); > - VM_BUG_ON(!PAGE_ALIGNED(end)); > - VM_BUG_ON_VMA(start < vma->vm_start, vma); > - VM_BUG_ON_VMA(end > vma->vm_end, vma); > + VM_WARN_ON_ONCE(!PAGE_ALIGNED(start)); > + VM_WARN_ON_ONCE(!PAGE_ALIGNED(end)); > + VM_WARN_ON_ONCE_VMA(start < vma->vm_start, vma); > + VM_WARN_ON_ONCE_VMA(end > vma->vm_end, vma); > mmap_assert_locked(mm); > > /* > @@ -1957,8 +1953,8 @@ long faultin_page_range(struct mm_struct *mm, unsigned long start, > int gup_flags; > long ret; > > - VM_BUG_ON(!PAGE_ALIGNED(start)); > - VM_BUG_ON(!PAGE_ALIGNED(end)); > + VM_WARN_ON_ONCE(!PAGE_ALIGNED(start)); > + VM_WARN_ON_ONCE(!PAGE_ALIGNED(end)); > mmap_assert_locked(mm); > > /* > @@ -2908,7 +2904,8 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, > } else if (pte_special(pte)) > goto pte_unmap; > > - VM_BUG_ON(!pfn_valid(pte_pfn(pte))); > + /* If it's not marked as special it must have a valid memmap. */ > + VM_WARN_ON_ONCE(!pfn_valid(pte_pfn(pte))); > page = pte_page(pte); > > folio = try_grab_folio_fast(page, 1, flags); > > base-commit: 2d0c297637e7d59771c1533847c666cdddc19884 > -- > 2.49.0 -- Michal Hocko SUSE Labs