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2N/Zh3m3pCCQfpsP25Gfs1OjZCXNdnPWtatzjlJ+TF0= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Jun 04, 2025 at 04:58:25PM +0200, David Hildenbrand wrote: > On 04.06.25 16:48, Lorenzo Stoakes wrote: > > +Linus in case he has an opinion about BUG_ON() in general... > > > > On Wed, Jun 04, 2025 at 04:05:44PM +0200, 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. > > > > I guess the situation where you'd actually want a BUG_ON() is one where > > carrying on might cause further corruption so you just want things to stop. > > Yes. Like, serious data corruption would be avoidable. Yeah, I just wonder how often this is ever reliably for sure the case... > > > > > But usually we're already pretty screwed if the thing happened right? So > > it's rare if ever that this would be legit? > > > > Linus's point of view is that we shouldn't use them _at all_ right? So > > maybe even this situation isn't one where we'd want to use one? > > I think the grey zone is actual data corruption. But one has to have a > pretty good reason to use a BUG_ON and not a WARN_ON_ONCE() + recovery. Right. > > > > > > > > > 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. > > > > Yeah VM_BUG_ON() is just _weird_. Maybe we should get rid of all of them > > full stop? > > That's my thinking a well. :) > > > > > > > > > We have to introduce VM_WARN_ON_ONCE_VMA() to make that fly. > > > > I checked the implementation vs. the other VM_WARN_ON_ONCE_*()'s and it > > looks good. > > > > I wonder if we can find a way to not duplicate this code... but one for a > > follow up I think :>) > > > > > > > > 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. > > > > > > 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 > > > > LGTM so, > > > > Reviewed-by: Lorenzo Stoakes > > > > > > Thanks! > > > One nit below. > > > > > --- > > > > > > Wanted to do this for a long time, but my todo list keeps growing ... > > > > Sounds familiar :) Merge window a chance to do some of these things... > > > > > > > > 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); \ > > > +}) > > > > An aside, I wonder if we could somehow make this generic for various > > WARN_ON_ONCE()'s? > > Yeah, probably. Maybe it will get .... ugly :) > > > > > > #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); > > > > Nit but wouldn't VM_WARN_ON_ONCE_FOLIO() work better here? > > No, we want the actual problematic page here, as that can give us clues what > is going wrong. Ah yeah... didn't notice we're checking both folio and page... PageAnonExclusive() seems to be a weird beast: /* * HugeTLB stores this information on the head page; THP keeps it per * page */ But anyway I'm digressing :) > > For the small-folio case above we could use it, though. Ack, no big deal though. > > -- > Cheers, > > David / dhildenb >