linux-kernel - Re: [RFC] a question about mlockall() and mprotect()

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Message-ID: <20170926081716.xo375arjoyu5ytcb@dhcp22.suse.cz>
Date:   Tue, 26 Sep 2017 10:17:16 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Xishi Qiu <qiuxishi@...wei.com>
Cc:     Joonsoo Kim <iamjoonsoo.kim@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        zhong jiang <zhongjiang@...wei.com>,
        yeyunfeng <yeyunfeng@...wei.com>, wanghaitao12@...wei.com,
        "Zhoukang (A)" <zhoukang7@...wei.com>
Subject: Re: [RFC] a question about mlockall() and mprotect()

On Tue 26-09-17 15:56:55, Xishi Qiu wrote:
> When we call mlockall(), we will add VM_LOCKED to the vma,
> if the vma prot is ---p,

not sure what you mean here. apply_mlockall_flags will set the flag on
all vmas except for special mappings (mlock_fixup). This phase will
cause that memory reclaim will not free already mapped pages in those
vmas (see page_check_references and the lazy mlock pages move to
unevictable LRUs).

> then mm_populate -> get_user_pages will not alloc memory.

mm_populate all the vmas with pages. Well there are certainly some
constrains - e.g. memory cgroup hard limit might be hit and so the
faulting might fail.

> I find it said "ignore errors" in mm_populate()
> static inline void mm_populate(unsigned long addr, unsigned long len)
> {
> 	/* Ignore errors */
> 	(void) __mm_populate(addr, len, 1);
> }

But we do not report the failure because any failure past
apply_mlockall_flags would be tricky to handle. We have already dropped
the mmap_sem lock so some other address space operations could have
interfered.

> And later we call mprotect() to change the prot, then it is
> still not alloc memory for the mlocked vma.
> 
> My question is that, shall we alloc memory if the prot changed,
> and who(kernel, glibc, user) should alloc the memory?

I do not understand your question but if you are asking how to get pages
to map your vmas then touching that area will fault the memory in.
-- 
Michal Hocko
SUSE Labs