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From:   Jason Gunthorpe <jgg@nvidia.com>
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Subject: Re: [PATCH v3 3/6] mm: support THP migration to device private memory
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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