From 1096bc93df75577d34db66e8b88ae6d0d1e72b1d Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Fri, 29 Mar 2024 11:06:13 -0700 Subject: mm: clean up populate_vma_page_range() FOLL_* flag handling The code wasn't exactly wrong, but it was very odd, and it used FOLL_FORCE together with FOLL_WRITE when it really didn't need to (it only set FOLL_WRITE for writable mappings, so then the FOLL_FORCE was pointless). It also pointlessly called __get_user_pages() even when it knew it wouldn't populate anything because the vma wasn't accessible and it explicitly tested for and did *not* set FOLL_FORCE for inaccessible vma's. This code does need to use FOLL_FORCE, because we want to do fault in writable shared mappings, but then the mapping may not actually be readable. And we don't want to use FOLL_WRITE (which would match the permission of the vma), because that would also dirty the pages, which we don't want to do. For very similar reasons, FOLL_FORCE populates a executable-only mapping with no read permissions. We don't have a FOLL_EXEC flag. Yes, it would probably be cleaner to split FOLL_WRITE into two bits (for separate permission and dirty bit handling), and add a FOLL_EXEC flag for the "GUP executable page" case. That would allow us to avoid FOLL_FORCE entirely here. But that's not how our FOLL_xyz bits have traditionally worked, and that would be a much bigger patch. So this at least avoids the FOLL_FORCE | FOLL_WRITE combination that made one of my experimental validation patches trigger a warning. That warning was a false positive (and my experimental patch was incomplete anyway), but it all made me look at this and decide to clean at least this small case up. Signed-off-by: Linus Torvalds --- mm/gup.c | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-) (limited to 'mm') diff --git a/mm/gup.c b/mm/gup.c index df83182ec72d5..af8edadc05d1b 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1653,20 +1653,22 @@ long populate_vma_page_range(struct vm_area_struct *vma, if (vma->vm_flags & VM_LOCKONFAULT) return nr_pages; + /* ... similarly, we've never faulted in PROT_NONE pages */ + if (!vma_is_accessible(vma)) + return -EFAULT; + gup_flags = FOLL_TOUCH; /* * We want to touch writable mappings with a write fault in order * to break COW, except for shared mappings because these don't COW * and we would not want to dirty them for nothing. + * + * Otherwise, do a read fault, and use FOLL_FORCE in case it's not + * readable (ie write-only or executable). */ if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE) gup_flags |= FOLL_WRITE; - - /* - * We want mlock to succeed for regions that have any permissions - * other than PROT_NONE. - */ - if (vma_is_accessible(vma)) + else gup_flags |= FOLL_FORCE; if (locked) -- cgit 1.2.3-korg