linux-kernel - Re: [patch for-5.3 0/4] revert immediate fallback to remote hugepages

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Date:   Thu, 3 Oct 2019 10:00:08 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Rientjes <rientjes@...gle.com>,
        Michal Hocko <mhocko@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [patch for-5.3 0/4] revert immediate fallback to remote hugepages

On 10/3/19 12:32 AM, David Rientjes wrote:
> On Wed, 2 Oct 2019, Michal Hocko wrote:
> 
>>>> If 
>>>> hugetlb wants to stress this to the fullest extent possible, it already 
>>>> appropriately uses __GFP_RETRY_MAYFAIL.
>>>
>>> Which doesn't work anymore right now, and should again after this patch.
>>
>> I didn't get to fully digest the patch Vlastimil is proposing. (Ab)using
>> __GFP_NORETRY is quite subtle but it is already in place with some
>> explanation and a reference to THPs. So while I am not really happy it
>> is at least something you can reason about.
>>
> 
> It's a no-op:
> 
>         /* Do not loop if specifically requested */
>         if (gfp_mask & __GFP_NORETRY)
>                 goto nopage;
> 
>         /*
>          * Do not retry costly high order allocations unless they are
>          * __GFP_RETRY_MAYFAIL
>          */
>         if (costly_order && !(gfp_mask & __GFP_RETRY_MAYFAIL))
>                 goto nopage;
> 
> So I'm not sure we should spend too much time discussing a hunk of a patch 
> that doesn't do anything.

I believe Michal was talking about my (ab)use of __GFP_NORETRY, where it
controls the earlier 'goto nopage' condition. Yes, with your patches alone,
the addition of __GFP_NORETRY in the second attempt is a no-op, although
then I don't see the point of confusing people reading the code with it.