linux-kernel - Re: [RFC V2 11/12] mm: Tag VMA with VM

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Message-ID: <abc1d8b7-cde0-ac40-6664-c10694666659@intel.com>
Date:   Tue, 31 Jan 2017 09:54:02 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     mhocko@...e.com, vbabka@...e.cz, mgorman@...e.de,
        minchan@...nel.org, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, srikar@...ux.vnet.ibm.com,
        haren@...ux.vnet.ibm.com, jglisse@...hat.com,
        dan.j.williams@...el.com
Subject: Re: [RFC V2 11/12] mm: Tag VMA with VM_CDM flag during page fault

On 01/30/2017 09:10 PM, Anshuman Khandual wrote:
>> This is happening with mmap_sem held for read.  Correct?  Is it OK that
>> you're modifying the VMA?  That vm_flags manipulation is non-atomic, so
>> how can that even be safe?
> Hmm. should it be done with mmap_sem being held for write. Will look
> into this further. But intercepting the page faults inside alloc_pages_vma()
> for tagging the VMA is okay from over all design perspective ?. Or this
> should be moved up or down the call chain in the page fault path ?

Doing it in the fault path seems wrong to me.

Apps have to take *explicit* action to go and get access to device
memory.  It seems like we should mark the VMA *then*, at the time of the
explicit action.  I also think _implying_ that we want KSM, etc...
turned off just because of the target of an mbind() is a bad idea.  Apps
have to ask for this stuff *explicitly*, so why not also have them turn
KSM off explicitly?