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

From: Mike Kravetz <mike.kravetz@oracle.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Michal Hocko <mhocko@kernel.org>, Hugh Dickins <hughd@google.com>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@linux.vnet.ibm.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	stable@vger.kernel.org
Subject: Re: [PATCH] hugetlbfs: dirty pages as they are added to pagecache
Date: Thu, 18 Oct 2018 21:50:32 -0700	[thread overview]
Message-ID: <9eb7cf71-5bc8-5cd4-9f3e-95375d40060b@oracle.com> (raw)
In-Reply-To: <20181018184726.fb8da5c733da5e0c6a235101@linux-foundation.org>

On 10/18/18 6:47 PM, Andrew Morton wrote:
> On Thu, 18 Oct 2018 20:46:21 -0400 Andrea Arcangeli <aarcange@redhat.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