Re: [PATCH] mm/fadvise: discard partial pages iff endbyte is also eof

[åå(Caspar)]

On 2018/1/4 19:34, Mel Gorman wrote:
> On Thu, Jan 04, 2018 at 02:13:43PM +0800, ??????(Caspar) wrote:
> > On 2018/1/3 18:48, Mel Gorman wrote:
> > > On Wed, Jan 03, 2018 at 02:53:43PM +0800, ??????(Caspar) wrote:
> > > > > ?? 2017??12??23????12:16?????? <shidao.ytt@xxxxxxxxxxxxxxx> ??????
> > > > >
> > > > > From: "shidao.ytt" <shidao.ytt@xxxxxxxxxxxxxxx>
> > > > >
> > > > > in commit 441c228f817f7 ("mm: fadvise: document the
> > > > > fadvise(FADV_DONTNEED) behaviour for partial pages") Mel Gorman
> > > > > explained why partial pages should be preserved instead of discarded
> > > > > when using fadvise(FADV_DONTNEED), however the actual codes to calcuate
> > > > > end_index was unexpectedly wrong, the code behavior didn't match to the
> > > > > statement in comments; Luckily in another commit 18aba41cbf
> > > > > ("mm/fadvise.c: do not discard partial pages with POSIX_FADV_DONTNEED")
> > > > > Oleg Drokin fixed this behavior
> > > > >
> > > > > Here I come up with a new idea that actually we can still discard the
> > > > > last parital page iff the page-unaligned endbyte is also the end of
> > > > > file, since no one else will use the rest of the page and it should be
> > > > > safe enough to discard.
> > > >
> > > > +akpm...
> > > >
> > > > Hi Mel, Andrew:
> > > >
> > > > Would you please take a look at this patch, to see if this proposal
> > > > is reasonable enough, thanks in advance!
> > >
> > > I'm backlogged after being out for the Christmas. Superficially the patch
> > > looks ok but I wondered how often it happened in practice as we already
> > > would discard files smaller than a page on DONTNEED. It also requires
> >
> > Actually, we would *not*. Let's look into the codes.
>
> You're right of course. I suggest updating the changelog with what you
> found and the test case. I think it's reasonable to special case the
> discarding of partial pages if it's the end of a file with the potential
> addendum of checking if the endbyte is past the end of the file. The man
> page should also be updated.

Sure, will do and send out v2.

Thanks,
Caspar
>