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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Oct 08, 2025 at 12:37:47PM +0800, Lance Yang wrote: > From: Lance Yang > > As Zi pointed out, we should avoid crashing the kernel for conditions > that can be handled gracefully. Encountering a non-anonymous folio in an > anonymous VMA is a bug, but a warning is sufficient. > > This patch changes the VM_BUG_ON_FOLIO(!folio_test_anon(folio)) to a > VM_WARN_ON_FOLIO() in both __collapse_huge_page_isolate() and > hpage_collapse_scan_pmd(), and then aborts the scan with SCAN_PAGE_ANON. Well no, in hpage_collapse_scan_pmd() there is no warning at all. > > Making more of the scanning logic common between hpage_collapse_scan_pmd() > and __collapse_huge_page_isolate(), as suggested by Dev. I mean I guess it's fine but I'm not sure it's really necessary to give a blow-by-blow of who suggested what in the actual commit message :) This isn't really useful information for somebody looking at this code in the future. > > Suggested-by: Dev Jain > Suggested-by: Zi Yan > Reviewed-by: Wei Yang > Reviewed-by: Dev Jain > Signed-off-by: Lance Yang > --- > mm/khugepaged.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index e3e27223137a..b5c0295c3414 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -573,7 +573,11 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma, > } > > folio = page_folio(page); > - VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio); > + if (!folio_test_anon(folio)) { > + VM_WARN_ON_FOLIO(true, folio); > + result = SCAN_PAGE_ANON; > + goto out; Hmm this is iffy I'm not sure I agree with Zi here - the purpose of VM_WARN_ON() etc. is for things that programmatically _should not happen_. Now every single time we run this code we're doing this check. AND it implies that it is an actual possiblity, at run time, for this to be the case. I really don't like this. Also if it's a runtime check this should be a WARN_ON_ONCE() not a VM_WARN_ON_ONCE(). Of course you lose the folio output then. So this code is very confused. In general I don't think we should be doing this at all, rather we should just convert the VM_BUG_ON_FOLIO() to a VM_WARN_ON_FOLIO(). > + } > > /* See hpage_collapse_scan_pmd(). */ > if (folio_maybe_mapped_shared(folio)) { > @@ -1340,6 +1344,7 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm, > folio = page_folio(page); > > if (!folio_test_anon(folio)) { > + VM_WARN_ON_FOLIO(true, folio); Err, what? This is a condition that should never, ever happen to the point that we warn on it? This surely is a condition that we expect to happen sometimes otherwise we wouldn't do this no? Either way the comments above still apply. Also VM_WARN_ON_FOLIO(true, ...) is kinda gross... if this is an actual pattern that exists, VM_WARN_FOLIO() would be preferable. > result = SCAN_PAGE_ANON; > goto out_unmap; > } > -- > 2.49.0 >