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Date: Tue, 15 Feb 2022 22:01:00 -0400 From: Jason Gunthorpe <jgg@...dia.com> To: Felix Kuehling <felix.kuehling@....com> Cc: Christoph Hellwig <hch@....de>, David Hildenbrand <david@...hat.com>, Alex Sierra <alex.sierra@....com>, akpm@...ux-foundation.org, linux-mm@...ck.org, rcampbell@...dia.com, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org, jglisse@...hat.com, apopple@...dia.com, willy@...radead.org Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support On Tue, Feb 15, 2022 at 05:49:07PM -0500, Felix Kuehling wrote: > > Userspace does > > 1) mmap(MAP_PRIVATE) to allocate anon memory > > 2) something to trigger migration to install a ZONE_DEVICE page > > 3) munmap() > > > > Who decrements the refcout on the munmap? > > > > When a ZONE_DEVICE page is installed in the PTE is supposed to be > > marked as pte_devmap and that disables all the normal page refcounting > > during munmap(). > > > > fsdax makes this work by working the refcounts backwards, the page is > > refcounted while it exists in the driver, when the driver decides to > > remove it then unmap_mapping_range() is called to purge it from all > > PTEs and then refcount is decrd. munmap/fork/etc don't change the > > refcount. > > Hmm, that just means, whether or not there are PTEs doesn't really > matter. Yes, that is the FSDAX model > It should work the same as it does for DEVICE_PRIVATE pages. I'm not sure > where DEVICE_PRIVATE page's refcounts are decremented on unmap, TBH. But I > can't find it in our driver, or in the test_hmm driver for that matter. It is not the same as DEVICE_PRIVATE because DEVICE_PRIVATE uses swap entries. The put_page for that case is here: static unsigned long zap_pte_range(struct mmu_gather *tlb, struct vm_area_struct *vma, pmd_t *pmd, unsigned long addr, unsigned long end, struct zap_details *details) { [..] if (is_device_private_entry(entry) || is_device_exclusive_entry(entry)) { struct page *page = pfn_swap_entry_to_page(entry); if (unlikely(zap_skip_check_mapping(details, page))) continue; pte_clear_not_present_full(mm, addr, pte, tlb->fullmm); rss[mm_counter(page)]--; if (is_device_private_entry(entry)) page_remove_rmap(page, false); put_page(page); However the devmap case will return NULL from vm_normal_page() and won't do the put_page() embedded inside the __tlb_remove_page() in the pte_present() block in the same function. After reflecting for awhile, I think Christoph's idea is quite good. Just make it so you don't set pte_devmap() on your pages and then lets avoid pte_devmap for all refcount correct ZONE_DEVICE pages. Jason
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