Re: [PATCHv2, RFC 08/30] thp, mm: rewrite add_to_page_cache_locked()to support huge pages

From: Dave Hansen
Date: Fri Mar 22 2013 - 10:50:19 EST

On 03/22/2013 03:34 AM, Kirill A. Shutemov wrote:
> Dave Hansen wrote:
>> On 03/14/2013 10:50 AM, Kirill A. Shutemov wrote:
>>> + error = radix_tree_insert(&mapping->page_tree,
>>> + offset + i, page + i);
>>> + if (error) {
>>> + page_cache_release(page + i);
>>> + break;
>>> + }
>>> }
>> Throughout all this new code, I'd really challenge you to try as much as
>> possible to minimize the code stuck under "if (PageTransHuge(page))".
> I put thp-related code under the 'if' intentionally to be able to optimize
> it out if !CONFIG_TRANSPARENT_HUGEPAGE. The config option is disabled by
> default.

You've gotta give the compiler some credit! :) In this function's case,
the compiler should be able to realize that nr=1 is constant if
!TRANSPARENT_HUGEPAGE. It'll realize that all your for loops are:

for (i = 0; i < 1; i++) {

and will end up generating _very_ similar code to what you get with the
explicit #ifdefs. You already _have_ #ifdefs, but they're just up in
the headers around PageTransHuge()'s definition.

The advantages for you are *huge* since it means that folks will have a
harder time inadvertently breaking your CONFIG_TRANSPARENT_HUGEPAGE code.

>> Does the cgroup code know how to handle these large pages internally
>> somehow? It looks like the charge/uncharge is only being done for the
>> head page.
> It can. We only need to remove PageCompound() check there. Patch is in
> git.

OK, cool. This _might_ deserve a comment or something here. Again, it
looks asymmetric and fishy to someone just reading the code.

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