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SubjectRe: [RFC PATCH 2/4] mm, tree wide: replace __GFP_REPEAT by __GFP_RETRY_MAYFAIL with more useful semantic
On Wed 07-06-17 10:10:36, Wei Yang wrote:
[...]
> Hmm... Let me be more specific. With two factors, costly or not, flag set or
> not, we have four combinations. Here it is classified into two categories.
>
> 1. __GFP_RETRY_MAYFAIL not set
>
> Brief description on behavior:
> costly: pick up the shortcut, so no OOM
> !costly: no shortcut and will OOM I think
>
> Impact from this patch set:
> No.

true

> My personal understanding:
> The allocation without __GFP_RETRY_MAYFAIL is not effected by this patch
> set. Since !costly allocation will trigger OOM, this is the reason why
> "small allocations never fail _practically_", as mentioned in
> https://lwn.net/Articles/723317/.
>
>
> 3. __GFP_RETRY_MAYFAIL set
>
> Brief description on behavior:
> costly/!costly: no shortcut here and no OOM invoked
>
> Impact from this patch set:
> For those allocations with __GFP_RETRY_MAYFAIL, OOM is not invoked for
> both.

yes

> My personal understanding:
> This is the semantic you are willing to introduce in this patch set. By
> cutting off the OOM invoke when __GFP_RETRY_MAYFAIL is set, you makes this
> a middle situation between NOFAIL and NORETRY.

yes

> page_alloc will try some luck to get some free pages without disturb other
> part of the system. By doing so, the never fail allocation for !costly
> pages will be "fixed". If I understand correctly, you are willing to make
> this the default behavior in the future?

I do not think we can make this a default in a foreseeable future
unfortunately. That's why I've made it a gfp modifier in the first
place. I assume many users will opt in by using the flag. In future we
can even help by adding a highlevel GFP_$FOO flag but I am worried that
this would just add to the explosion of existing highlevel gfp masks
(e.g. do we want GFP_NOFS_MAY_FAIL, GFP_USER_MAY_FAIL,
GFP_USER_HIGH_MOVABLE_MAYFAIL etc...)
--
Michal Hocko
SUSE Labs

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