Re: [PATCH] mm/util.c: Added page count to __vm_enough_memory failure warning

[Matt Cassell <mcassell411@gmail.com>]

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Thank you for the feedback. I agree with you and would prefer to use bytes/kbytes. Here are the 2 concerns that led to me keeping it as pages:


1. Reduce the impact of the patch. Here is the call trace to reach the failure warning:

	<… usual mmap() stuff …>
	mmap_region() -> security_enough_memory_mm() -> __vm_enough_memory()

	Within mmap_region(), the length variable originally passed to mmap() gets right-shifted to get the page count. My first thought was to add an additional an additional argument to security_enough_memory_mm() of type 	unsigned long to keep that variable, but saw a handful of calls to it that would have to conform to the change. Not that I do not think this debug statement does not warrant that, I felt the less impact, the better.


2. Concerned about losing bits. When converting back to bytes I was worried about the loss of precision and printing that number back to users:

	unsigned long bytes_failed = pages << (PAGE_SHIFT);


> On Feb 22, 2024, at 6:18 AM, David Hildenbrand <david@redhat.com> wrote:
> 
> On 21.02.24 17:02, Matthew Cassell wrote:
>> Commit 44b414c8715c5dcf53288 ("mm/util.c: add warning if __vm_enough_memory
>> fails") adds debug information which gives the process id and executable name
>> should __vm_enough_memory() fail. Adding the number of pages to the failure
>> message would benefit application developers and system administrators in
>> debugging overambitious memory requests by providing a point of reference to
>> the amount of memory causing __vm_enough_memory() to fail.
>> 1. Set appropriate kernel tunable to reach code path for failure
>>    message:
>> 	# echo 2 > /proc/sys/vm/overcommit_memory
>> 2. Test program to generate failure - requests 1 gibibyte per iteration:
>> 	#include <stdlib.h>
>> 	#include <stdio.h>
>> 	int main(int argc, char **argv) {
>> 		for(;;) {
>> 			if(malloc(1<<30) == NULL)
>> 				break;
>> 			printf("allocated 1 GiB\n");
>> 		}
>> 		return 0;
>> 	}
>> 3. Output:
>> 	Before:
>> 	__vm_enough_memory: pid: 1218, comm: a.out, not enough
>> memory for the allocation
>> 	After:
>> 	__vm_enough_memory: pid: 1141, comm: a.out, pages: 262145, not
>> enough memory for the allocation
>> Signed-off-by: Matthew Cassell <mcassell411@gmail.com>
>> ---
>>  mm/util.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> diff --git a/mm/util.c b/mm/util.c
>> index 5a6a9802583b..c0afb56f16ea 100644
>> --- a/mm/util.c
>> +++ b/mm/util.c
>> @@ -976,8 +976,8 @@ int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin)
>>  	if (percpu_counter_read_positive(&vm_committed_as) < allowed)
>>  		return 0;
>>  error:
>> -	pr_warn_ratelimited("%s: pid: %d, comm: %s, not enough memory for the allocation\n",
>> -			    __func__, current->pid, current->comm);
>> +	pr_warn_ratelimited("%s: pid: %d, comm: %s, pages: %ld, not enough memory for the allocation\n",
>> +			    __func__, current->pid, current->comm, pages);
>>  	vm_unacct_memory(pages);
>>    	return -ENOMEM;
> 
> I wonder if "bytes"/"kbytes" instead of pages would be more appropriate here.
> 
> Often, this will fail due to mmap() [where we pass a size from user space] and also "vm.overcommit_kbytes" is not in pages.
> 
> -- 
> Cheers,
> 
> David / dhildenb


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