Messages in this thread | | | Subject | Re: [PATCH v1 2/5] mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault/prealloc memory | From | David Hildenbrand <> | Date | Tue, 30 Mar 2021 18:30:48 +0200 |
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On 30.03.21 18:21, Jann Horn wrote: > On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand <david@redhat.com> wrote: >>>> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start, >>>> + unsigned long end, bool write, int *locked) >>>> +{ >>>> + struct mm_struct *mm = vma->vm_mm; >>>> + unsigned long nr_pages = (end - start) / PAGE_SIZE; >>>> + int gup_flags; >>>> + >>>> + 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); >>>> + mmap_assert_locked(mm); >>>> + >>>> + /* >>>> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit >>>> + * a poisoned page. >>>> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT. >>>> + * !FOLL_FORCE: Require proper access permissions. >>>> + */ >>>> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON; >>>> + if (write) >>>> + gup_flags |= FOLL_WRITE; >>>> + >>>> + /* >>>> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings >>>> + * or with insufficient permissions. >>>> + */ >>>> + return __get_user_pages(mm, start, nr_pages, gup_flags, >>>> + NULL, NULL, locked); >>> >>> You mentioned in the commit message that you don't want to actually >>> dirty all the file pages and force writeback; but doesn't >>> POPULATE_WRITE still do exactly that? In follow_page_pte(), if >>> FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty: >> >> Well, I mention that POPULATE_READ explicitly doesn't do that. I >> primarily set it because populate_vma_page_range() also sets it. >> >> Is it safe to *not* set it? IOW, fault something writable into a page >> table (where the CPU could dirty it without additional page faults) >> without marking it accessed? For me, this made logically sense. Thus I >> also understood why populate_vma_page_range() set it. > > FOLL_TOUCH doesn't have anything to do with installing the PTE - it > essentially means "the caller of get_user_pages wants to read/write > the contents of the returned page, so please do the same things you > would do if userspace was accessing the page". So in particular, if > you look up a page via get_user_pages() with FOLL_WRITE|FOLL_TOUCH, > that tells the MM subsystem "I will be writing into this page directly > from the kernel, bypassing the userspace page tables, so please mark > it as dirty now so that it will be properly written back later". Part > of that is that it marks the page as recently used, which has an > effect on LRU pageout behavior, I think - as far as I understand, that > is why populate_vma_page_range() uses FOLL_TOUCH. > > If you look at __get_user_pages(), you can see that it is split up > into two major parts: faultin_page() for creating PTEs, and > follow_page_mask() for grabbing pages from PTEs. faultin_page() > ignores FOLL_TOUCH completely; only follow_page_mask() uses it. > > In a way I guess maybe you do want the "mark as recently accessed" > part that FOLL_TOUCH would give you without FOLL_WRITE? But I think > you very much don't want the dirtying that FOLL_TOUCH|FOLL_WRITE leads > to. Maybe the ideal approach would be to add a new FOLL flag to say "I > only want to mark as recently used, I don't want to dirty". Or maybe > it's enough to just leave out the FOLL_TOUCH entirely, I don't know.
Any thoughts why populate_vma_page_range() does it?
-- Thanks,
David / dhildenb
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