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SubjectRe: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap_thread()
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On Fri, 2020-10-09 at 14:34 -0700, Eric Biggers wrote:
> On Fri, Oct 09, 2020 at 12:49:57PM -0700, ira.weiny@intel.com wrote:
> > From: Ira Weiny <ira.weiny@intel.com>
> >
> > The kmap() calls in this FS are localized to a single thread. To
> > avoid the over head of global PKRS updates use the new
> > kmap_thread() call.
> >
> > Cc: Jaegeuk Kim <jaegeuk@kernel.org>
> > Cc: Chao Yu <chao@kernel.org>
> > Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> > ---
> > fs/f2fs/f2fs.h | 8 ++++----
> > 1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index d9e52a7f3702..ff72a45a577e 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -2410,12 +2410,12 @@ static inline struct page
> > *f2fs_pagecache_get_page(
> >
> > static inline void f2fs_copy_page(struct page *src, struct page
> > *dst)
> > {
> > - char *src_kaddr = kmap(src);
> > - char *dst_kaddr = kmap(dst);
> > + char *src_kaddr = kmap_thread(src);
> > + char *dst_kaddr = kmap_thread(dst);
> >
> > memcpy(dst_kaddr, src_kaddr, PAGE_SIZE);
> > - kunmap(dst);
> > - kunmap(src);
> > + kunmap_thread(dst);
> > + kunmap_thread(src);
> > }
>
> Wouldn't it make more sense to switch cases like this to
> kmap_atomic()?
> The pages are only mapped to do a memcpy(), then they're immediately
> unmapped.

On a VIPT/VIVT architecture, this is horrendously wasteful. You're
taking something that was mapped at colour c_src mapping it to a new
address src_kaddr, which is likely a different colour and necessitates
flushing the original c_src, then you copy it to dst_kaddr, which is
also likely a different colour from c_dst, so dst_kaddr has to be
flushed on kunmap and c_dst has to be invalidated on kmap. What we
should have is an architectural primitive for doing this, something
like kmemcopy_arch(dst, src). PIPT architectures can implement it as
the above (possibly losing kmap if they don't need it) but VIPT/VIVT
architectures can set up a correctly coloured mapping so they can
simply copy from c_src to c_dst without any need to flush and the data
arrives cache hot at c_dst.

James


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