Re: [PATCH v12 23/31] mm: don't do swap readahead during speculative page fault

[Laurent Dufour]

Le 22/04/2019 Ã 23:36, Jerome Glisse a ÃcritÂ:
> On Tue, Apr 16, 2019 at 03:45:14PM +0200, Laurent Dufour wrote:
> > Vinayak Menon faced a panic because one thread was page faulting a page in
> > swap, while another one was mprotecting a part of the VMA leading to a VMA
> > split.
> > This raise a panic in swap_vma_readahead() because the VMA's boundaries
> > were not more matching the faulting address.
> >
> > To avoid this, if the page is not found in the swap, the speculative page
> > fault is aborted to retry a regular page fault.
> >
> > Reported-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
> > Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
>
> Reviewed-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
>
> Note that you should also skip non swap entry in do_swap_page() when doing
> speculative page fault at very least you need to is_device_private_entry()
> case.
>
> But this should either be part of patch 22 or another patch to fix swap
> case.

Thanks JÃrÃme,

Yes I missed that, I guess the best option would be to abort on non swap 
entry. I'll add that in the patch 22.

> > ---
> >   mm/memory.c | 11 +++++++++++
> >   1 file changed, 11 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 6e6bf61c0e5c..1991da97e2db 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2900,6 +2900,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >   				lru_cache_add_anon(page);
> >   				swap_readpage(page, true);
> >   			}
> > +		} else if (vmf->flags & FAULT_FLAG_SPECULATIVE) {
> > +			/*
> > +			 * Don't try readahead during a speculative page fault
> > +			 * as the VMA's boundaries may change in our back.
> > +			 * If the page is not in the swap cache and synchronous
> > +			 * read is disabled, fall back to the regular page
> > +			 * fault mechanism.
> > +			 */
> > +			delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
> > +			ret = VM_FAULT_RETRY;
> > +			goto out;
> >   		} else {
> >   			page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
> >   						vmf);
> > --
> > 2.21.0