Re: [PATCH 5.10] fget: clarify and improve __fget_files() implementation

[libaokun (A)]

在 2022/2/18 2:55, Greg KH 写道:
> On Tue, Feb 15, 2022 at 02:51:07PM +0800, Baokun Li wrote:
> > From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> >
> > commit e386dfc56f837da66d00a078e5314bc8382fab83 upstream.
> >
> > Commit 054aa8d439b9 ("fget: check that the fd still exists after getting
> > a ref to it") fixed a race with getting a reference to a file just as it
> > was being closed.  It was a fairly minimal patch, and I didn't think
> > re-checking the file pointer lookup would be a measurable overhead,
> > since it was all right there and cached.
> >
> > But I was wrong, as pointed out by the kernel test robot.
> >
> > The 'poll2' case of the will-it-scale.per_thread_ops benchmark regressed
> > quite noticeably.  Admittedly it seems to be a very artificial test:
> > doing "poll()" system calls on regular files in a very tight loop in
> > multiple threads.
> >
> > That means that basically all the time is spent just looking up file
> > descriptors without ever doing anything useful with them (not that doing
> > 'poll()' on a regular file is useful to begin with).  And as a result it
> > shows the extra "re-check fd" cost as a sore thumb.
> >
> > Happily, the regression is fixable by just writing the code to loook up
> > the fd to be better and clearer.  There's still a cost to verify the
> > file pointer, but now it's basically in the noise even for that
> > benchmark that does nothing else - and the code is more understandable
> > and has better comments too.
> >
> > [ Side note: this patch is also a classic case of one that looks very
> >    messy with the default greedy Myers diff - it's much more legible with
> >    either the patience of histogram diff algorithm ]
> >
> > Link: https://lore.kernel.org/lkml/20211210053743.GA36420@xsang-OptiPlex-9020/
> > Link: https://lore.kernel.org/lkml/20211213083154.GA20853@xxxxxxxxxxxxxxx/
> > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> > Tested-by: Carel Si <beibei.si@xxxxxxxxx>
> > Cc: Jann Horn <jannh@xxxxxxxxxx>
> > Cc: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> > ---
> >   fs/file.c | 72 ++++++++++++++++++++++++++++++++++++++++++-------------
> >   1 file changed, 56 insertions(+), 16 deletions(-)
> Now queued up, thanks.
Thanks!
> Any chance you can do this for 5.4 and older kernels too?
It's my pleasure. I'll sync this patch to 5.4, 4.19, 4.14, 4.9, 4.4.
> greg k-h
> .

--
With Best Regards,
Baokun Li

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