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That was > undesired, because follow_page() is usually only used to lookup a currently > mapped page, not to actually access it. Further, follow_page() does not > actually trigger fault handling, but instead simply fails. I see that follow_page() is also completely undocumented. And that reduces us to deducing how it should be used...these things that change follow_page()'s behavior maybe should have a go at documenting it too, perhaps. > > Let's restore that behavior by conditionally setting FOLL_FORCE if > FOLL_WRITE is not set. This way, for example KSM and migration code will > no longer fail on PROT_NONE mapped PTEs/PMDS. > > Handling this internally doesn't require us to add any new FOLL_FORCE > usage outside of GUP code. > > While at it, refuse to accept FOLL_FORCE: we don't even perform VMA > permission checks like in check_vma_flags(), so especially > FOLL_FORCE|FOLL_WRITE would be dodgy. > > This issue was identified by code inspection. We'll add some > documentation regarding FOLL_FORCE next. > > Reported-by: Peter Xu > Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()") > Cc: > Signed-off-by: David Hildenbrand > --- > mm/gup.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 2493ffa10f4b..da9a5cc096ac 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -841,9 +841,17 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, > if (vma_is_secretmem(vma)) > return NULL; > > - if (WARN_ON_ONCE(foll_flags & FOLL_PIN)) > + if (WARN_ON_ONCE(foll_flags & (FOLL_PIN | FOLL_FORCE))) > return NULL; This is not a super happy situation: follow_page() is now prohibited (see above: we should document that interface) from passing in FOLL_FORCE... > > + /* > + * Traditionally, follow_page() succeeded on PROT_NONE-mapped pages > + * but failed follow_page(FOLL_WRITE) on R/O-mapped pages. Let's > + * keep these semantics by setting FOLL_FORCE if FOLL_WRITE is not set. > + */ > + if (!(foll_flags & FOLL_WRITE)) > + foll_flags |= FOLL_FORCE; > + ...but then we set it anyway, for special cases. It's awkward because FOLL_FORCE is not an "internal to gup" flag (yet?). I don't yet have suggestions, other than: 1) Yes, the FOLL_NUMA made things bad. 2) And they are still very confusing, especially the new use of FOLL_FORCE. ...I'll try to let this soak in and maybe recommend something in a more productive way. :) thanks, -- John Hubbard NVIDIA