linux-kernel - Re: [PATCH v4 00/36] Large pages in the page cache

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Message-ID: <20200522025751.GX2005@dread.disaster.area>
Date:   Fri, 22 May 2020 12:57:51 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 00/36] Large pages in the page cache

On Thu, May 21, 2020 at 05:04:11PM -0700, Matthew Wilcox wrote:
> On Fri, May 22, 2020 at 08:49:06AM +1000, Dave Chinner wrote:
> > Ok, so the main issue I have with the filesystem/iomap side of
> > things is that it appears to be adding "transparent huge page"
> > awareness to the filesysetm code, not "large page support".
> > 
> > For people that aren't aware of the difference between the
> > transparent huge and and a normal compound page (e.g. I have no idea
> > what the difference is), this is likely to cause problems,
> > especially as you haven't explained at all in this description why
> > transparent huge pages are being used rather than bog standard
> > compound pages.
> 
> The primary reason to use a different name from compound_*
> is so that it can be compiled out for systems that don't enable
> CONFIG_TRANSPARENT_HUGEPAGE.  So THPs are compound pages, as they always
> have been, but for a filesystem, using thp_size() will compile to either
> page_size() or PAGE_SIZE depending on CONFIG_TRANSPARENT_HUGEPAGE.

Again, why is this dependent on THP? We can allocate compound pages
without using THP, so why only allow the page cache to use larger
pages when THP is configured?

i.e. I don't know why this is dependent on THP because you haven't
explained why this only works for THP and not just plain old
compound pages....

> Now, maybe thp_size() is the wrong name, but then you need to suggest
> a better name ;-)

First you need to explain why THP is requirement for large pages in
the page cache when most of the code changes I see only care if the
page is a compound page or not....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com