On Tue, 2015-06-23 at 14:44 +0300, Kirill A. Shutemov wrote: > On Mon, Jun 22, 2015 at 02:55:55PM -0600, Toshi Kani wrote: > > On Sat, 2015-06-20 at 22:46 +0300, Kirill A. Shutemov wrote: > > > On Tue, Jun 16, 2015 at 04:28:30PM -0600, Toshi Kani wrote: > > > > DAX has the following issues in a shared or read-only private > > > > mmap'd file. > > > > - mmap(MAP_POPULATE) does not pre-fault > > > > - mlock() fails with -ENOMEM > > > > > > > > DAX uses VM_MIXEDMAP for mmap'd files, which do not have struct > > > > page associated with the ranges. Both MAP_POPULATE and mlock() > > > > call __mm_populate(), which in turn calls __get_user_pages(). > > > > Because __get_user_pages() requires a valid page returned from > > > > follow_page_mask(), MAP_POPULATE and mlock(), i.e. FOLL_POPULATE, > > > > fail in the first page. > > > > > > > > Change __get_user_pages() to proceed FOLL_POPULATE when the > > > > translation is set but its page does not exist (-EFAULT), and > > > > @pages is not requested. With that, MAP_POPULATE and mlock() > > > > set translations to the requested range and complete successfully. > > > > > > > > MAP_POPULATE still provides a major performance improvement to > > > > DAX as it will avoid page faults during initial access to the > > > > pages. > > > > > > > > mlock() continues to set VM_LOCKED to vma and populate the range. > > > > Since there is no struct page, the range is pinned without marking > > > > pages mlocked. > > > > > > > > Note, MAP_POPULATE and mlock() already work for a write-able > > > > private mmap'd file on DAX since populate_vma_page_range() breaks > > > > COW, which allocates page caches. > > > > > > I don't think that's true in all cases. > > > > > > We would fail to break COW for mlock() if the mapping is populated with > > > read-only entries by the mlock() time. In this case follow_page_mask() > > > would fail with -EFAULT and faultin_page() will never executed. > > > > No, mlock() always breaks COW as populate_vma_page_range() sets > > FOLL_WRITE in case of write-able private mmap. > > > > /* > > * 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. > > */ > > if ((vma->vm_flags & (VM_WRITE | VM_SHARED)) == VM_WRITE) > > gup_flags |= FOLL_WRITE; > > Okay, you're right it should work. > > What about doing this in more generic way? The totally untested patch > below tries to make GUP work on DAX and other pfn maps when struct page > is not required. > > Any comments? The changes look good to me. I have also run my mmap() & mlock() tests and they passed with the changes. (They can only exercise follow_pfn_pte() with FOLL_TOUCH, though.) Reviewed-by: Toshi Kani <toshi.kani@xxxxxx> Thanks, -Toshi -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
