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fork: define a local GFP_VMAP_STACK 

The current allocation of VMAP stack memory is using (THREADINFO_GFP &
~__GFP_ACCOUNT) which is a complicated way of saying (GFP_KERNEL |
__GFP_ZERO):

<linux/thread_info.h>:
define THREADINFO_GFP (GFP_KERNEL_ACCOUNT | __GFP_ZERO)
<linux/gfp_types.h>:
define GFP_KERNEL_ACCOUNT (GFP_KERNEL | __GFP_ACCOUNT)

This is an unfortunate side-effect of independent changes blurring the
picture:

commit 19809c2da2 changed (THREADINFO_GFP |
__GFP_HIGHMEM) to just THREADINFO_GFP since highmem became implicit.

commit 9b6f7e163c then added stack caching
and rewrote the allocation to (THREADINFO_GFP & ~__GFP_ACCOUNT) as cached
stacks need to be accounted separately.  However that code, when it
eventually accounts the memory does this:

  ret = memcg_kmem_charge(vm->pages[i], GFP_KERNEL, 0)

so the memory is charged as a GFP_KERNEL allocation.

Define a unique GFP_VMAP_STACK to use
GFP_KERNEL | __GFP_ZERO and move the comment there.

Link: https://lkml.kernel.org/r/20250509-gfp-stack-v1-1-82f6f7efc210@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reported-by: Mateusz Guzik <mjguzik@gmail.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Linus Walleij 2025-05-09 09:25:09 +02:00 committed by Andrew Morton
parent 449e0b4ed5
commit f7b0ff2bc9
1 changed files with 7 additions and 6 deletions
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13
kernel/fork.c
View File

@ -201,6 +201,12 @@ static inline void free_task_struct(struct task_struct *tsk)
*/
#define NR_CACHED_STACKS 2
static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);
/*
* Allocated stacks are cached and later reused by new threads, so memcg
* accounting is performed by the code assigning/releasing stacks to tasks.
* We need a zeroed memory without __GFP_ACCOUNT.
*/
#define GFP_VMAP_STACK (GFP_KERNEL | __GFP_ZERO)
struct vm_stack {
struct rcu_head rcu;
@ -307,13 +313,8 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node)
return 0;
}
/*
* Allocated stacks are cached and later reused by new threads,
* so memcg accounting is performed manually on assigning/releasing
* stacks to tasks. Drop __GFP_ACCOUNT.
*/
stack = __vmalloc_node(THREAD_SIZE, THREAD_ALIGN,
THREADINFO_GFP & ~__GFP_ACCOUNT,
GFP_VMAP_STACK,
node, __builtin_return_address(0));
if (!stack)
return -ENOMEM;