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SubjectRe: does the semantics of MAP_LOCKED is equal to mlock() function?
See http://marc.info/?l=linux-mm&m=142122902313315&w=2 for more
information.

On Mon 19-01-15 14:17:43, long.wanglong wrote:
> Hi all:
>
> In the latest kernel, i set the memory limit (4096) in a test cgroup. and add the test task.
> the test code code is:
>
> testcase 1: mmap with MAP_LOCKED flag(memsize = 8192)
>
> 185 p = mmap(NULL, memsize, PROT_WRITE | PROT_READ,
> 186 MAP_PRIVATE | MAP_ANONYMOUS | MAP_LOCKED, 0, 0);
> 187 if (p == MAP_FAILED)
> 188 err(1, "mmap(lock) failed");
>
> expect: invoke OOM killer.
> result: not invoke OOM killer.
>
>
> testcase 2: mmap without MAP_LOCKED flag and the call mlock (memsize = 8192)
>
> 185 p = mmap(NULL, memsize, PROT_WRITE | PROT_READ,
> 186 MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
> 187 if (p == MAP_FAILED)
> 188 err(1, "mmap(lock) failed");
> 189
> 190 if (mlock(p, memsize) == -1)
> 191 err(1, "mlock failed")
>
> expect: invoke OOM killer.
> result: invoke OOM killer.
>
> in the Linux Programmer's Manual:
>
> MAP_LOCKED (since Linux 2.5.37)
> Lock the pages of the mapped region into memory in the manner
> of mlock(2). This flag is ignored in older kernels.
>
> and
> mlock() locks pages in the address range starting at addr and
> continuing for len bytes. All pages that contain a part of the
> specified address range are guaranteed to be resident in RAM when the
> call returns successfully; the pages are guaranteed to stay in RAM
> until later unlocked.
>
> According to the description in the manual, the two testcases are equivalent.
>
> why the first testcase does not invoke OOM killer?
> does the mmap with MAP_LOCKED flag will not immediately allocate physical memory?
>
>
> Best Regards
> Wang Long
>

--
Michal Hocko
SUSE Labs


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