Re: [PATCHv9-rebased2 01/37] mm, thp: make swapin readahead under down_read of mmap_sem

[Ebru Akagunduz <ebru.akagunduz@gmail.com>]

On Mon, Jun 20, 2016 at 10:51:25AM +0800, Hillf Danton wrote:
> > > > > @@ -2401,11 +2430,18 @@ static void __collapse_huge_page_swapin(struct mm_struct *mm,
> > > > >  			continue;
> > > > >  		swapped_in++;
> > > > >  		ret = do_swap_page(mm, vma, _address, pte, pmd,
> > > > > -				   FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT,
> > > > > +				   FAULT_FLAG_ALLOW_RETRY,
> > > >
> > > > Add a description in change log for it please.
> > >
> > > Ebru, would you address it?
> > >
> > This changelog really seems poor.
> > Is there a way to update only changelog of the commit?
> > I tried to use git rebase to amend commit, however
> > I could not rebase. This patch only needs better changelog.
> > 
> > I would like to update it as follows, if you would like to too:
> > 
> > "
> > Currently khugepaged makes swapin readahead under down_write.  This patch
> > supplies to make swapin readahead under down_read instead of down_write.
> > 
> > Along swapin, we can need to drop and re-take mmap_sem. Therefore we
> > have to be sure vma is consistent. This patch adds a helper function
> > to validate vma and also supplies that async swapin should not be
> > performed without waiting.
> > 
> > The patch was tested with a test program that allocates 800MB of memory,
> > writes to it, and then sleeps.  The system was forced to swap out all.
> > Afterwards, the test program touches the area by writing, it skips a page
> > in each 20 pages of the area.
> > "
> > 
> I like to ask again, why is FAULT_FLAG_RETRY_NOWAIT dropped?
> 
I dropped it regarding to Hugh's concern. If I understood correctly,
he said async swapin without waiting can cause waste. When I looked the
code path, it seemed subtle to me. There was a large discussion,
below is Hugh's concern:

"
Doesn't this imply that __collapse_huge_page_swapin() will initiate all
the necessary swapins for a THP, then (given the FAULT_FLAG_ALLOW_RETRY)
not wait for them to complete, so khugepaged will give up on that extent
and move on to another; then after another full circuit of all the mms
it needs to examine, it will arrive back at this extent and build a THP
from the swapins it arranged last time.

Which may work well when a system transitions from busy+swappingout
to idle+swappingin, but isn't that rather a special case?  It feels
(meaning, I've not measured at all) as if the inbetween busyish case
will waste a lot of I/O and memory on swapins that have to be discarded
again before khugepaged has made its sedate way back to slotting them in.
"

Cc'ed Hugh. Maybe I misunderstood him.

> > Could you please suggest me a way to replace above changelog with the old?
> > 
> We can ask Andrew for some advices.
> 
> > > >
> > > > They are cleaned up in subsequent darns?
> > >
> > Yes, that is reported and fixed here:
> > http://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=fc7038a69cee6b817261f7cd805e9663fdc1075c
> > 
> > However, the above comment inconsistency still there.
> > I've added a fix patch:
> > 
> > From 404438ff1b0617cbf7434cba0c5a08f79ccb8a5d Mon Sep 17 00:00:00 2001
> > From: Ebru Akagunduz <ebru.akagunduz@gmail.com>
> > Date: Sat, 18 Jun 2016 21:07:22 +0300
> > Subject: [PATCH] mm, thp: fix comment inconsistency for swapin readahead
> >  functions
> >
> Fill in change log please.
> 
I filled it and sent as part of a series.
Cc'ed you in that patch.
> thanks
> Hillf
>  
> > Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
> > ---
> >  mm/huge_memory.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index acd374e..f0d528e 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2436,9 +2436,10 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> >  		/* do_swap_page returns VM_FAULT_RETRY with released mmap_sem */
> >  		if (ret & VM_FAULT_RETRY) {
> >  			down_read(&mm->mmap_sem);
> > -			/* vma is no longer available, don't continue to swapin */
> > -			if (hugepage_vma_revalidate(mm, address))
> > +			if (hugepage_vma_revalidate(mm, address)) {
> > +				/* vma is no longer available, don't continue to swapin */
> >  				return false;
> > +			}
> >  		}
> >  		if (ret & VM_FAULT_ERROR) {
> >  			trace_mm_collapse_huge_page_swapin(mm, swapped_in, 0);
> > @@ -2513,8 +2514,8 @@ static void collapse_huge_page(struct mm_struct *mm,
> >  	if (allocstall == curr_allocstall && swap != 0) {
> >  		/*
> >  		 * __collapse_huge_page_swapin always returns with mmap_sem
> > -		 * locked.  If it fails, release mmap_sem and jump directly
> > -		 * out.  Continuing to collapse causes inconsistency.
> > +		 * locked. If it fails, we release mmap_sem and jump out_nolock.
> > +		 * Continuing to collapse causes inconsistency.
> >  		 */
> >  		if (!__collapse_huge_page_swapin(mm, vma, address, pmd)) {
> >  			mem_cgroup_cancel_charge(new_page, memcg, true);
> > --
> > 1.9.1
> 

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