Re: Is get_user_pages() enough to prevent pages from being swappedout ?

[Hugh Dickins]

On Wed, 29 Jul 2009, Laurent Pinchart wrote:
> 
> I'm trying to debug a video acquisition device driver and found myself having 
> to dive deep into the memory management subsystem.
> 
> The driver uses videobuf-dma-sg to manage video buffers. videobuf-dma-sg gets 
> pointers to buffers from userspace and calls get_user_pages() to retrieve the 
> list of pages underlying those buffers. The page list is used to build a 
> scatter-gather list that is given to the hardware. The device then performs 
> DMA directly to the memory.
> 
> Pages underlying the buffers must obviously not be swapped out during DMA.
> The get_user_pages() (mm/memory.c) documentation seems to imply that returned 
> pages are pinned to memory (my understanding of "pinned" is that they will
> not be swapped out):
...
> However, all is seems to do for that purpose is incrementing the page 
> reference count using get_page().
> 
> I had a look through the memory management subsystem code and it seems to me 
> that incrementing the reference count is not sufficient to make sure the page 
> won't be swapped out. To ensure that, it should instead be marked as 
> unevictable, either directly or by marking an associated VMA as VM_LOCKED. 
> This is what the mlock() syscall does, in addition to calling 
> get_user_pages().
> 
> The MM subsystem is quite complex and my understanding might not be correct, 
> so I'd appreciate if someone could shed light on the issue. Does 
> get_user_pages() really pin pages to memory and prevent them from being 
> swapped out in all circumstances ? If so, how does it do so ? If not, what's 
> the proper way to make sure the pages won't disappear during DMA ?

You're right that get_user_pages() (called with a pagelist as you're
using) increments the page reference count.

And that is enough to pin the page in memory, in a sense that suits
the use of DMA.

I'm expressing it in that peculiar way, because:- On the one hand,
the page can only disappear from memory by memory hotremove, but
what you'll be worrying about is the page getting freed and reused
for another purpose while DMA is acting upon it - but raising the
reference count prevents that (and will prevent hotremove succeeding).

On the other hand, despite the raised reference count, under memory
pressure that page might get unmapped from the user pagetable, and
might even be written out to swap in its half-dirty state (though
is_page_cache_freeable() tries to avoid that); but it won't get
freed, and DMA will be to the physical address of the page (somebody
will correct me that it's actually the bus address or something else),
not to the userspace virtual address.  So it's irrelevant if that
vanishes for a while - when userspace accesses it again, the same
page (the one DMA occurs to) will be faulted back in there.

In contrast, mlock() is not enough to pin a page in memory in this
sense: from a userspace point of view, an mlock()ed page indeed is
locked into memory, but page migration (for NUMA balancing or for
memory hotremove) is still free to substitute an alternative page
there.  get_user_pages()'s raised reference count prevents that,
but mlock() does not.

There is one little problem with the get_user_pages() pinning,
hopefully one that can never affect you at all.  If the task that
did the get_user_pages() forks, and parent or child userspace tries
to write to one of the pages in question while it's pinned (and it's
an anonymous page, not a pagecache page shared with underlying file),
then the first to touch it will get a copy of the original DMA page
at that instant, thereafter losing contact with the original DMA page.

One answer to that is to madvise such an area with MADV_DONTFORK,
then fork won't duplicate that area, so no Copy-on-Write issues
will arise.  That satisfies many of us, but others look for a
way to eliminate this issue completely.

Hugh
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