linux-kernel - Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG

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Message-ID: <5A7D771C-FF95-465E-95F6-CD249FE28381@vmware.com>
Date:   Sun, 19 Dec 2021 06:02:00 +0000
From:   Nadav Amit <namit@...are.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>
CC:     Jason Gunthorpe <jgg@...dia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)

> On Dec 18, 2021, at 4:35 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> (I have only ever seen the kernel side of uffd, not the actual user
> side, so I'm not sure about the use patterns).

I use it in a very fine granularity, and I suspect QEMU and CRIU do so
too.

> 
> That said, your suggestion of a shadow sw page table bit thing would
> also work. And it would solve some problems we have in core areas
> (notably "page_special()" which right now has that
> ARCH_HAS_PTE_SPECIAL thing).
> 
> It would make it really easy to have that "this page table entry is
> pinned" flag too.

I found my old messy code for the software-PTE thing.

I see that eventually I decided to hold a pointer to the “extra PTEs”
of each page in the PMD-page-struct. [ I also implemented the 2-adjacent
pages approach but this code is long gone. ]

My rationale was that:

1. It does not bound you to have the same size for PTE and “extra-PTE”
2. The PMD-page struct is anyhow hot (since you acquired the PTL)
3. Allocating “extra-PTE” dynamically does not require to rewire the
   page-tables, which requires a TLB flush.

I think there is a place to hold a pointer in the PMD-page-struct
(_pt_pad_1, we just need to keep the lowest bit clear so the kernel
won’t mistaken it to be a compound page).

I still don’t know what exactly you have in mind for making use
out of it for the COW issue. Keeping a pin-count (which requires
internal API changes for unpin_user_page() and friends?) or having
“was ever pinned” sticky bit? And then changing
page_needs_cow_for_dma() to look at the PTE so copy_present_pte()
would break the COW eagerly?

Anyhow, I can clean it up and send (although it is rather simple
and I ignored many thing, such as THP, remap, etc), but I am not
sure I have the time now to fully address the COW problem. I will
wait for Monday for David’s response.