linux-kernel - Re: [PATCH] hugetlbfs: dirty pages as they are added to pagecache

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Date:   Thu, 18 Oct 2018 21:50:32 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        stable@...r.kernel.org
Subject: Re: [PATCH] hugetlbfs: dirty pages as they are added to pagecache

On 10/18/18 6:47 PM, Andrew Morton wrote:
> On Thu, 18 Oct 2018 20:46:21 -0400 Andrea Arcangeli <aarcange@...hat.com> wrote:
> 
>> On Thu, Oct 18, 2018 at 04:16:40PM -0700, Mike Kravetz wrote:
>>> I was not sure about this, and expected someone could come up with
>>> something better.  It just seems there are filesystems like huegtlbfs,
>>> where it makes no sense wasting cycles traversing the filesystem.  So,
>>> let's not even try.
>>>
>>> Hoping someone can come up with a better method than hard coding as
>>> I have done above.
>>
>> It's not strictly required after marking the pages dirty though. The
>> real fix is the other one? Could we just drop the hardcoding and let
>> it run after the real fix is applied?

Yeah.  The other part of the patch is the real fix.  This drop_caches
part is not necessary.

>> The performance of drop_caches doesn't seem critical, especially with
>> gigapages. tmpfs doesn't seem to be optimized away from drop_caches
>> and the gain would be bigger for tmpfs if THP is not enabled in the
>> mount, so I'm not sure if we should worry about hugetlbfs first.
> 
> I guess so.  I can't immediately see a clean way of expressing this so
> perhaps it would need a new BDI_CAP_NO_BACKING_STORE.  Such a
> thing hardly seems worthwhile for drop_caches.
> 
> And drop_caches really shouldn't be there anyway.  It's a standing
> workaround for ongoing suckage in pagecache and metadata reclaim
> behaviour :(

I'm OK with dropping the other part.  It just seemed like there was no
real reason to try and drop_caches for hugetlbfs (and perhaps others).

Andrew, would you like another version?  Or can you just drop the
fs/drop_caches.c part?

-- 
Mike Kravetz