linux-kernel - Re: [PATCH] mm/util.c: Added page count to __vm_enough

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Message-ID: <CANiscBBhk+g267yYz8DhgU23gt0Ranak4VctxeG1-_tt_KkOag@mail.gmail.com>
Date: Thu, 22 Feb 2024 10:24:16 -0600
From: Matthew Cassell <mcassell411@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/util.c: Added page count to __vm_enough_memory failure warning

Resending due to plain-text email issue that caused mailing list to get skipped.

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 Thu, Feb 22, 2024 at 6:18 AM David Hildenbrand <david@...hat.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@...il.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
>