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SubjectRe: [RFC PATCH] Add proc interface to set PF_MEMALLOC flags
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On 2019/09/11 12:13, Hillf Danton wrote:
>
> On Tue, 10 Sep 2019 11:06:03 -0500 From: Mike Christie <mchristi@redhat.com>
>>
>>> Really? Without any privilege check? So any random user can tap into
>>> __GFP_NOIO allocations?
>>
>> That was a mistake on my part. I will add it in.
>>
> You may alternatively madvise a nutcracker as long as you would have
> added a sledgehammer under /proc instead of a gavel.
>
> --- a/include/uapi/asm-generic/mman-common.h
> +++ b/include/uapi/asm-generic/mman-common.h
> @@ -45,6 +45,7 @@
> #define MADV_SEQUENTIAL 2 /* expect sequential page references */
> #define MADV_WILLNEED 3 /* will need these pages */
> #define MADV_DONTNEED 4 /* don't need these pages */
> +#define MADV_NOIO 5 /* set PF_MEMALLOC_NOIO */
>
> /* common parameters: try to keep these consistent across architectures */
> #define MADV_FREE 8 /* free pages only if memory pressure */
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -716,6 +716,7 @@ madvise_behavior_valid(int behavior)
> case MADV_WILLNEED:
> case MADV_DONTNEED:
> case MADV_FREE:
> + case MADV_NOIO:
> #ifdef CONFIG_KSM
> case MADV_MERGEABLE:
> case MADV_UNMERGEABLE:
> @@ -813,6 +814,11 @@ SYSCALL_DEFINE3(madvise, unsigned long,
> if (!madvise_behavior_valid(behavior))
> return error;
>
> + if (behavior == MADV_NOIO) {
> + current->flags |= PF_MEMALLOC_NOIO;

Yes, for "modifying p->flags when p != current" is not permitted.

But I guess that there is a problem. Setting PF_MEMALLOC_NOIO causes
current_gfp_context() to mask __GFP_IO | __GFP_FS, but the OOM killer cannot
be invoked when __GFP_FS is masked. As a result, any userspace thread which
has PF_MEMALLOC_NOIO cannot invoke the OOM killer. If the userspace thread
which uses PF_MEMALLOC_NOIO is involved in memory reclaiming activities,
the memory reclaiming activities won't be able to make forward progress when
the userspace thread triggered e.g. a page fault. Can the "userspace components
that can run in the IO path" survive without any memory allocation?

> + return 0;
> + }
> +
> if (start & ~PAGE_MASK)
> return error;
> len = (len_in + ~PAGE_MASK) & PAGE_MASK;

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