Re: [RFC 0/6] mm: support madvise(MADV_FREE)

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 18, 2014 at 05:23:37PM -0700, Andy Lutomirski wrote:
> On Tue, Mar 18, 2014 at 5:18 PM, Minchan Kim <minchan@kernel.org> wrote:
> > Hello,
> >
> > On Tue, Mar 18, 2014 at 10:55:24AM -0700, Andy Lutomirski wrote:
> >> On 03/13/2014 11:37 PM, Minchan Kim wrote:
> >> > This patch is an attempt to support MADV_FREE for Linux.
> >> >
> >> > Rationale is following as.
> >> >
> >> > Allocators call munmap(2) when user call free(3) if ptr is
> >> > in mmaped area. But munmap isn't cheap because it have to clean up
> >> > all pte entries, unlinking a vma and returns free pages to buddy
> >> > so overhead would be increased linearly by mmaped area's size.
> >> > So they like madvise_dontneed rather than munmap.
> >> >
> >> > "dontneed" holds read-side lock of mmap_sem so other threads
> >> > of the process could go with concurrent page faults so it is
> >> > better than munmap if it's not lack of address space.
> >> > But the problem is that most of allocator reuses that address
> >> > space soonish so applications see page fault, page allocation,
> >> > page zeroing if allocator already called madvise_dontneed
> >> > on the address space.
> >> >
> >> > For avoidng that overheads, other OS have supported MADV_FREE.
> >> > The idea is just mark pages as lazyfree when madvise called
> >> > and purge them if memory pressure happens. Otherwise, VM doesn't
> >> > detach pages on the address space so application could use
> >> > that memory space without above overheads.
> >>
> >> I must be missing something.
> >>
> >> If the application issues MADV_FREE and then writes to the MADV_FREEd
> >> range, the kernel needs to know that the pages are no longer safe to
> >> lazily free.  This would presumably happen via a page fault on write.
> >> For that to happen reliably, the kernel has to write protect the pages
> >> when MADV_FREE is called, which in turn requires flushing the TLBs.
> >
> > It could be done by pte_dirty bit check. Of course, if some architectures
> > don't support it by H/W, pte_mkdirty would make it CoW as you said.
> 
> If the page already has dirty PTEs, then you need to clear the dirty
> bits and flush TLBs so that other CPUs notice that the PTEs are clean,
> I think.
> 
> Also, this has very odd semantics wrt reading the page after MADV_FREE
> -- is reading the page guaranteed to un-free it?

MADV_FREE simply invalidates content.  Sure, you can read at a given
address repeatedly after it.  You might see a different page every
time you do it, but it doesn't matter; the content is undefined.

It's no different than doing malloc() and looking at the memory before
writing anything in it.  After MADV_FREE, the memory is like a freshly
malloc'd chunk: the first access may result in page faults and the
content is undefined until you write it.

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