linux-kernel - Re: [PATCH v2 13/25] fs: Add zero_user

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Date:   Fri, 21 Feb 2020 15:42:40 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 13/25] fs: Add zero_user_large

On Tue, Feb 18, 2020 at 10:07:05AM -0800, Matthew Wilcox wrote:
> On Tue, Feb 18, 2020 at 08:10:52PM +0300, Kirill A. Shutemov wrote:
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> 
> Thanks
> 
> > > +#if defined(CONFIG_HIGHMEM) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
> > > +void zero_user_segments(struct page *page, unsigned start1, unsigned end1,
> > > +               unsigned start2, unsigned end2);
> > > +#else /* !HIGHMEM || !TRANSPARENT_HUGEPAGE */
> > 
> > This is a neat trick. I like it.
> > 
> > Although, it means non-inlined version will never get tested :/
> 
> I worry about that too, but I don't really want to incur the overhead on
> platforms people actually use.

I'm also worried about latency: kmap_atomic() disables preemption even if
system has no highmem. Some archs have way too large THP to clear them
with preemption disabled.

I *think* there's no real need in preemption disabling in this situation
and we can wrap kmap_atomic()/kunmap_atomic() into CONFIG_HIGHMEM.

-- 
 Kirill A. Shutemov