linux-kernel - Re: [PATCH v1] kernel/trace:check the val against the available mem

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Date:   Thu, 5 Apr 2018 17:32:40 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Zhaoyang Huang <huangzhaoyang@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-patch-test@...ts.linaro.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v1] kernel/trace:check the val against the available mem

On Thu 05-04-18 08:13:59, Matthew Wilcox wrote:
> On Thu, Apr 05, 2018 at 04:27:49PM +0200, Michal Hocko wrote:
> > On Thu 05-04-18 07:22:58, Matthew Wilcox wrote:
> > > On Wed, Apr 04, 2018 at 09:12:52PM -0700, Joel Fernandes wrote:
> > > > On Wed, Apr 4, 2018 at 7:58 PM, Matthew Wilcox <willy@...radead.org> wrote:
> > > > > I still don't get why you want RETRY_MAYFAIL.  You know that tries
> > > > > *harder* to allocate memory than plain GFP_KERNEL does, right?  And
> > > > > that seems like the exact opposite of what you want.
> 
> Argh.  The comment confused me.  OK, now I've read the source and
> understand that GFP_KERNEL | __GFP_RETRY_MAYFAIL tries exactly as hard
> as GFP_KERNEL *except* that it won't cause OOM itself.  But any other
> simultaneous GFP_KERNEL allocation without __GFP_RETRY_MAYFAIL will
> cause an OOM.  (And that's why we're having a conversation)

Well, I can udnerstand how this can be confusing. The all confusion
boils down to the small-never-fails semantic we have. So all reclaim
modificators (__GFP_NOFAIL, __GFP_RETRY_MAYFAIL, __GFP_NORETRY) only
modify the _default_ behavior.

> That's a problem because we have places in the kernel that call
> kv[zm]alloc(very_large_size, GFP_KERNEL), and that will turn into vmalloc,
> which will do the exact same thing, only it will trigger OOM all by itself
> (assuming the largest free chunk of address space in the vmalloc area
> is larger than the amount of free memory).

well, hardcoded GFP_KERNEL from vmalloc guts is yet another, ehm,
herritage that you are not so proud of.
 
> I considered an alloc_page_array(), but that doesn't fit well with the
> design of the ring buffer code.  We could have:
> 
> struct page *alloc_page_list_node(int nid, gfp_t gfp_mask, unsigned long nr);
> 
> and link the allocated pages together through page->lru.
> 
> We could also have a GFP flag that says to only succeed if we're further
> above the existing watermark than normal.  __GFP_LOW (==ALLOC_LOW),
> if you like.  That would give us the desired behaviour of trying all of
> the reclaim methods that GFP_KERNEL would, but not being able to exhaust
> all the memory that GFP_KERNEL allocations would take.

Well, I would be really careful with yet another gfp mask. They are so
incredibly hard to define properly and then people kinda tend to screw
your best intentions with their usecases ;)
Failing on low wmark is very close to __GFP_NORETRY or even
__GFP_NOWAIT, btw. So let's try to not overthink this...
-- 
Michal Hocko
SUSE Labs