On Wed, Nov 11, 2020 at 03:38:42PM -0800, Ralph Campbell wrote: > MEMORY_DEVICE_GENERIC: > Struct pages are created in dev_dax_probe() and represent non-volatile memory. > The device can be mmap()'ed which calls dax_mmap() which sets > vma->vm_flags | VM_HUGEPAGE. > A CPU page fault will result in a PTE, PMD, or PUD sized page > (but not compound) to be inserted by vmf_insert_mixed() which will call either > insert_pfn() or insert_page(). > Neither insert_pfn() nor insert_page() increments the page reference > count. But why was this done? It seems very strange to put a pfn with a struct page into a VMA and then deliberately not take the refcount for the duration of that pfn being in the VMA? What prevents memunmap_pages() from progressing while VMAs still point at the memory? > I think just leaving the page reference count at one is better than trying > to use the mmu_interval_notifier or changing vmf_insert_mixed() and > invalidations of pfn_t_devmap(pfn) to adjust the page reference count. Why so? The entire point of getting struct page's for this stuff was to be able to follow the struct page flow. I never did learn a reason why there is devmap stuff all over the place in the page table code... > MEMORY_DEVICE_FS_DAX: > Struct pages are created in pmem_attach_disk() and virtio_fs_setup_dax() with > an initial reference count of one. > The problem I see is that there are 3 states that are important: > a) memory is free and not allocated to any file (page_ref_count() == 0). > b) memory is allocated to a file and in the page cache (page_ref_count() == 1). > c) some gup() or I/O has a reference even after calling unmap_mapping_pages() > (page_ref_count() > 1). ext4_break_layouts() basically waits until the > page_ref_count() == 1 with put_page() calling wake_up_var(&page->_refcount) > to wake up ext4_break_layouts(). > The current code doesn't seem to distinguish (a) and (b). If we want to use > the 0->1 reference count to signal (c), then the page cache would have hold > entries with a page_ref_count() == 0 which doesn't match the general page cache > assumptions. This explanation feels confusing. If *anything* has a reference on the page it cannot be recycled. I would have guess the logic is to remove it from the page cache then wait for a 0 reference?? Jason
