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SubjectRe: [RFC] Page table sharing
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On February 18, 2002 08:04 pm, Hugh Dickins wrote:
> On Mon, 18 Feb 2002, Daniel Phillips wrote:
> > On February 18, 2002 09:09 am, Hugh Dickins wrote:
> > > So how is the page_table_lock taken by swap_out effective when it's
> > > dealing with a page table shared by another mm than the one it is
> > > locking? And when handling a read-fault, again no such code (but
> > > when handling a write-fault, __pte_alloc has unshared in advance).
> > >
> > > Since copy_page_range would not copy shared page tables, I'm wrong to
> > > point there. But __pte_alloc does copy shared page tables (to unshare
> > > them), and needs them to be stable while it does so: so locking against
> > > swap_out really is required. It also needs locking against read faults,
> > > and they against each other: but there I imagine it's just a matter of
> > > dropping the write arg to __pte_alloc, going back to pte_alloc again.
> >
> > You're right about the read faults, wrong about swap_out. In general you've
> > been more right than wrong, so thanks. I'll post a new patch pretty soon and
> > I'd appreciate your comments.
>
> On the read faults: I see no change there in the patch you then posted,
> handle_mm_fault still calls __pte_alloc with write_access argument, so
> concurrent read faults on the same pte can still slot the page into the
> shared page table at the same time, doubly counting it.

Right. Oops. Let me contemplate for a moment.

> - no problem if
> it's the Reserved empty_zero_page, and I think no problem at present
> if it's a SwapCache page, since that is PageLocked in the current tree
> (but not in -aa, and in due course we should go Andrea's way there);
> but if it's a file page the double count will leave it unfreeable.
>
> On swap_out versus __pte_alloc: I was misreading it and you're almost
> right there: but you do need to change that "pte_t pte = *src_ptb;"
> to something atomic - hmm, do we have any primitive for doing that?

I guess we need one now.

> neither set_pte nor ptep_get_and_clear is right. Otherwise, on PAE
> HIGHMEM64G systems the two halves of "pte" could be assigned before
> and after try_to_swap_out's ptep_get_and_clear. But once you've got
> "pte", yes, you're basing all your decisions on your one local copy,
> that gives all the stability you need.

Thanks a lot, let me digest this and I'm close that hole shortly, or
feel free to suggest a fix. You just explained the memory leak I'm
seeing.

--
Daniel
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