linux-kernel - Re: [PATCH] mremap: Avoid TLB flushing anonymous pages that are not in swap cache

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Message-ID: <ecb75c29-3d1b-3b5e-ec9d-59c4f6c1ef08@intel.com>
Date:   Tue, 5 Jun 2018 12:49:18 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, mhocko@...nel.org,
        vbabka@...e.cz, Aaron Lu <aaron.lu@...el.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mremap: Avoid TLB flushing anonymous pages that are not
 in swap cache

On 06/05/2018 12:12 PM, Mel Gorman wrote:
> That's fair enough. I updated part of the changelog to read
> 
> This patch special cases anonymous pages to only flush ranges under the
> page table lock if the page is in swap cache and can be potentially queued
> for IO. Note that the full flush of the range being mremapped is still
> flushed so TLB flushes are not eliminated entirely.
> 
> Does that work for you?

Looks good, thanks.

>> I usually try to make the non-pte-modifying functions take a pte_t
>> instead of 'pte_t *' to make it obvious that there no modification going
>> on.  Any reason not to do that here?
> 
> No, it was just a minor saving on stack usage.

We're just splitting hairs now :) but, realistically, this little helper
will get inlined anyway, so it probably all generates the same code.

...
>> BTW, do you want to add a tiny comment about why we do the
>> trylock_page()?  I assume it's because we don't want to wait on finding
>> an exact answer: we just assume it is in the swap cache if the page is
>> locked and flush regardless.
> 
> It's really because calling lock_page while holding a spinlock is
> eventually going to ruin your day.

Hah, yeah, that'll do it.  Could you comment this, too?