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From: David Hildenbrand <>
To: Tiberiu Georgescu <>
Cc: Peter Xu <>,
	"" <>,
	"" <>,
	Alistair Popple <>,
	Ivan Teterevkov <>,
	Mike Rapoport <>,
	Hugh Dickins <>,
	Matthew Wilcox <>,
	Andrea Arcangeli <>,
	"Kirill A . Shutemov" <>,
	Andrew Morton <>,
	Mike Kravetz <>,
	"Carl Waldspurger [C]" <>,
	Florian Schmidt <>,
	Jonathan Davies <>
Subject: Re: [PATCH RFC 0/4] mm: Enable PM_SWAP for shmem with PTE_MARKER
Date: Thu, 19 Aug 2021 19:26:28 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 19.08.21 16:54, Tiberiu Georgescu wrote:
>> On 18 Aug 2021, at 19:13, David Hildenbrand <> wrote:
>>>>> I'm now wondering whether for Tiberiu's case mincore() can also be used.  It
>>>>> should just still be a bit slow because it'll look up the cache too, but it
>>>>> should work similarly like the original proposal.
>>> I am afraid that the information returned by mincore is a little too vague to be of better help, compared to what the pagemap should provide in theory. I will have a look to see whether lseek on
>>> proc/map_files works as a "PM_SWAP" equivalent. However, the swap offset would still be missing.
>> Well, with mincore() you could at least decide "page is present" vs. "page is swapped or not existent". At least for making pageout decisions it shouldn't really matter, no? madvise(MADV_PAGEOUT) on a hole is a nop.
> I think you are right. In the optimisation we first presented, we should be able to
> send the madvise(MADV_PAGEOUT) call even if the page is none quite safely
> and get the wanted behaviour. Also, the "is_present" or "is_swap_or_none"
> question can be answered by the current pagemap too. Nice catch.
> However, not all use cases are the same. AFAIK, there is still no way to figure
> out whether a shared page is swapped out or none unless it is directly
> read/accessed after a pagemap check. Bringing a page into memory to check
> if it previously was in swap does not seem ideal.

Well, you can lseek() to remove all the holes and use mincore() to 
remove all in-memory pages. You're left with the swapped ones. Not the 
most efficient interface maybe, but there is a way :)

> Also, we still have no mechanism to retrieve the swap offsets of shmem pages
> AFAIK. There is one more QEMU optimisation we are working on that requires
> these mappings available outside of kernel space.

How exactly would the swap offset really help? IMHO that's a kernel 
internal that shouldn't be of any value to user space -- it's merely for 
debugging purposes. But I'd love to learn details.


>> If it has an fd and we can punch that into syscalls, we should much rather use that fd to lookup stuff then going via process page tables -- if possible of course (to be evaluated, because I haven't looked into the CRIU details and how they use lseek with anonymous shared memory).
> I found out that it is possible to retrieve the fds of shmem/tmpfs file allocations
> using proc/pid/map_files, which is neat. Still, CRIU does not seem to care
> whether a page is swapped out or just empty, only if it is present on page cache.
> The holes that lseek finds would not be able to infer this difference, AFAIK. Will
> test the behaviour to make sure.

CRIU wants to migrate everything. lseek() gives you the definitive 
answer what needs migration -- if it's swapped out or resident. Just 
skip the holes.


David / dhildenb

  reply	other threads:[~2021-08-19 17:26 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-07  3:25 Peter Xu
2021-08-07  3:25 ` [PATCH RFC 1/4] mm: Introduce PTE_MARKER swap entry Peter Xu
2021-08-07  3:25 ` [PATCH RFC 2/4] mm: Check against orig_pte for finish_fault() Peter Xu
2021-08-07  3:25 ` [PATCH RFC 3/4] mm: Handle PTE_MARKER page faults Peter Xu
2021-08-07  3:25 ` [PATCH RFC 4/4] mm: Install marker pte when page out for shmem pages Peter Xu
2021-08-13 15:18   ` Tiberiu Georgescu
2021-08-13 16:01     ` Peter Xu
2021-08-18 18:02       ` Tiberiu Georgescu
2021-08-17  9:04 ` [PATCH RFC 0/4] mm: Enable PM_SWAP for shmem with PTE_MARKER David Hildenbrand
2021-08-17 17:09   ` Peter Xu
2021-08-17 18:46     ` David Hildenbrand
2021-08-17 20:24       ` Peter Xu
2021-08-18  8:24         ` David Hildenbrand
2021-08-18 17:52           ` Tiberiu Georgescu
2021-08-18 18:13             ` David Hildenbrand
2021-08-19 14:54               ` Tiberiu Georgescu
2021-08-19 17:26                 ` David Hildenbrand [this message]
2021-08-20 16:49                   ` Tiberiu Georgescu
2021-08-20 19:12                     ` Peter Xu
2021-08-25 13:40                       ` Tiberiu Georgescu
2021-08-25 14:59                         ` Peter Xu

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