linux-kernel - Re: [PATCH v2] mm/uffd: UFFD_FEATURE_WP

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Message-ID: <fad40511-3909-4362-5760-bcb57c94e534@collabora.com>
Date:   Thu, 2 Mar 2023 20:14:05 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        "kernel@...labora.com" <kernel@...labora.com>
Subject: Re: [PATCH v2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED

On 3/2/23 7:01 PM, David Hildenbrand wrote:
> On 02.03.23 14:57, Peter Xu wrote:
>> On Thu, Mar 02, 2023 at 10:37:44AM +0100, David Hildenbrand wrote:
>>> Especially for such large sparse VMAs, the current way of allocating
>>> pagetables to place markers/zeropages is far from optimal.
>>
>> IMHO that's not a generic workload.  As mentioned in the reply there, I
>> would suggest we go with simple then we have space to optimize it in the
>> future if necessary, because the API will be the same.
This is a good idea.

I'm trying to understand why aren't we going with most optimized
implementation. Why aren't we targeting it at this point in time?

>>
> 
> I disagree with "generic workload", we use sparse mmaps all over the place,
> and when blindly used by e.g., CRIU, we'll simply end up wasting memory and
> time.
I've heard about a use case where a file of size 10s of GBs can be mapped
to the memory and then accessed off and on. We need to handle this
correctly and efficiently.

> 
> But I already agreed that this optimization that is a separate thing to
> implement.
> 

-- 
BR,
Muhammad Usama Anjum