2.2/2.4/2.6 VMs: do malloc() ever return NULL?

Ihar 'Philips' Filipau
2003-11-25T13:27:22Z

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Hello!

I just wondering: do last three stable branches of LK able to return
malloc()==NULL and/or ENOMEM?

2.2: I cannot test this stuff right now - but it was hanging hard on
"for (;;) memset(malloc(N), 0, N);" So we do not have NULL from malloc().
2.4: same behaviour if OOM disabled. But by default (OOM even has no
configuration entry - so always on) it just kills offending process. No
NULL pointer either.
2.6: the same as 2.4 with oom killer (default conf). I have no test
system to check 2.6. w/o oom killer.

Resume: we malloc() never returns NULL. so man-pages are incorrect ;-)
[ AFAIK only kmalloc(GFP_ATOMIC) can potentially return NULL - but I
didn't yet tested my modules under memory pressure. TODO. ]

Most interesting thing is that on 2.4 "for(;;) malloc(N)" able to
allocate about 1.8GB of memory on system with only 256M of physical RAM.
Laghtingly fast. Then malloc() returns NULL. Good signs.
But as soon as I will put "memset()" - app will be killed or box
will go bananas. Neither of this two things are appropriate. Especially
when applications are tuned to handle memmory allocation errors.

Can anyone comment on this?

Does any one has patches with replacement VM? I'm very interesting in
knowing all aspect of VM<->block layer<->rest of the kernel integration.

Pointers to on-line memory management algorithms will be valuable
too, if some one knows any.

P.S. I know about linux-mm.org - out-dated, but I do read it right now.
Any other pointers will be appreciated.
But looks like general Linux crowd is satisfied with current (and past)
status of MM...

P.P.S. Small memory eating app is attached. lines 16 & 17 (for() loop)
used to touch every kbyte of memory to make it really allocated.

--
Ihar 'Philips' Filipau / with best regards from Saarbruecken.
-- _ _ _
Because the kernel depends on it existing. "init" |_|*|_|
literally _is_ special from a kernel standpoint, |_|_|*|
because its' the "reaper of zombies" (and, may I add, |*|*|*|
that would be a great name for a rock band).
-- Linus Torvalds

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#include <stdlib.h>
#include <stdio.h>

#define CHUNK_SIZE (1<<16)

int main()
{
char *b;
unsigned long sz = 0;
for (;;) {
fprintf(stderr, "alloc ");
if ((b = malloc( CHUNK_SIZE ))) {
unsigned int i;
sz += CHUNK_SIZE;
for (i=0; i<(CHUNK_SIZE>>10); ++i)
b[i<<10] = '0';
} else
exit(1);
fprintf(stderr, "done. [%lu%s]\n", sz>>10, "k");
}
}