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SubjectRe: [PATCH] mm: use managed_zone() for more exact check in zone iteration
On Thu, 15 Nov 2018 07:50:40 +0800 Wei Yang <richard.weiyang@gmail.com> wrote:

> For one zone, there are three digits to describe its space range:
>
> spanned_pages
> present_pages
> managed_pages
>
> The detailed meaning is written in include/linux/mmzone.h. This patch
> concerns about the last two.
>
> present_pages is physical pages existing within the zone
> managed_pages is present pages managed by the buddy system
>
> >From the definition, managed_pages is a more strict condition than
> present_pages.
>
> There are two functions using zone's present_pages as a boundary:
>
> populated_zone()
> for_each_populated_zone()
>
> By going through the kernel tree, most of their users are willing to
> access pages managed by the buddy system, which means it is more exact
> to check zone's managed_pages for a validation.
>
> This patch replaces those checks on present_pages to managed_pages by:
>
> * change for_each_populated_zone() to for_each_managed_zone()
> * convert for_each_populated_zone() to for_each_zone() and check
> populated_zone() where is necessary
> * change populated_zone() to managed_zone() at proper places
>
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>
> ---
>
> Michal, after last mail, I did one more thing to replace
> populated_zone() with managed_zone() at proper places.
>
> One thing I am not sure is those places in mm/compaction.c. I have
> chaged them. If not, please let me know.
>
> BTW, I did a boot up test with the patched kernel and looks smooth.

Seems sensible, but a bit scary. A basic boot test is unlikely to
expose subtle gremlins.

Worse, the situations in which managed_zone() != populated_zone() are
rare(?), so it will take a long time for problems to be discovered, I
expect.

I'll toss it in there for now, let's see who breaks :(

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