linux-kernel - Re: [HMM 12/15] mm/migrate: new memory migration helper for use with device memory v4

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Date:   Fri, 30 Jun 2017 16:19:25 -0700
From:   Evgeny Baskakov <ebaskakov@...dia.com>
To:     Jérôme Glisse <jglisse@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
CC:     John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        Mark Hairgrove <mhairgrove@...dia.com>,
        Sherry Cheung <SCheung@...dia.com>,
        Subhash Gutti <sgutti@...dia.com>
Subject: Re: [HMM 12/15] mm/migrate: new memory migration helper for use with
 device memory v4

On 6/26/17 5:07 PM, Evgeny Baskakov wrote:

 > Hi Jerome,
 >
 > The documentation shown above doesn't tell what the alloc_and_copy 
callback should do for source pages that have not been allocated yet. 
Instead, it unconditionally suggests checking if the MIGRATE_PFN_VALID 
and MIGRATE_PFN_MIGRATE flags are set.
 >
 > Based on my testing and looking in the source code, I see that for 
such pages the respective 'src' PFN entries are always set to 0 without 
any flags.
 >
 > The sample driver specifically handles that by checking if there's no 
page in the 'src' entry, and ignores any flags in such case:
 >
 >     struct page *spage = migrate_pfn_to_page(*src_pfns);
 >     ...
 >     if (spage && !(*src_pfns & MIGRATE_PFN_MIGRATE))
 >         continue;
 >
 >     if (spage && (*src_pfns & MIGRATE_PFN_DEVICE)) {
 >
 > I would like to suggest reflecting that in the documentation. Or, 
which would be more logical, migrate_vma could keep the zero in the PFN 
entries for not allocated pages, but set the MIGRATE_PFN_MIGRATE flag 
anyway.
 >
 > Thanks!
 >
 > Evgeny Baskakov
 > NVIDIA
 >

Hi Jerome,

It seems that the kernel can pass 0 in src_pfns for pages that it cannot 
migrate (i.e. the kernel knows that they cannot migrate prior to calling 
alloc_and_copy).

So, a zero in src_pfns can mean either "the page is not allocated yet" 
or "the page cannot migrate".

Can migrate_vma set the MIGRATE_PFN_MIGRATE flag for not allocated 
pages? On the driver side it is difficult to differentiate between the 
cases.

Thanks!

Evgeny Baskakov
NVIDIA