linux-ext4 - Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap

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Date:   Sun, 11 Oct 2020 23:56:35 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
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Subject: Re: [PATCH RFC PKS/PMEM 22/58] fs/f2fs: Utilize new kmap_thread()

On Fri, Oct 09, 2020 at 06:30:36PM -0700, Eric Biggers wrote:
> On Sat, Oct 10, 2020 at 01:39:54AM +0100, Matthew Wilcox wrote:
> > On Fri, Oct 09, 2020 at 02:34:34PM -0700, Eric Biggers wrote:
> > > On Fri, Oct 09, 2020 at 12:49:57PM -0700, ira.weiny@...el.com wrote:
> > > > 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.
> > > >
> > > > @@ -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.
> > 
> > Maybe you missed the earlier thread from Thomas trying to do something
> > similar for rather different reasons ...
> > 
> > https://lore.kernel.org/lkml/20200919091751.011116649@linutronix.de/
> 
> I did miss it.  I'm not subscribed to any of the mailing lists it was sent to.
> 
> Anyway, it shouldn't matter.  Patchsets should be standalone, and not require
> reading random prior threads on linux-kernel to understand.

Sorry, but I did not think that the discussion above was directly related.  If
I'm not mistaken, Thomas' work was directed at relaxing kmap_atomic() into
kmap_thread() calls.  While interesting, it is not the point of this series.  I
want to restrict kmap() callers into kmap_thread().

For this series it was considered to change the kmap_thread() call sites to
kmap_atomic().  But like I said in the cover letter kmap_atomic() is not the
same semantic.  It is too strict.  Perhaps I should have expanded that
explanation.

> 
> And I still don't really understand.  After this patchset, there is still code
> nearly identical to the above (doing a temporary mapping just for a memcpy) that
> would still be using kmap_atomic().

I don't understand.  You mean there would be other call sites calling:

kmap_atomic()
memcpy()
kunmap_atomic()

?

> Is the idea that later, such code will be
> converted to use kmap_thread() instead?  If not, why use one over the other?
 

The reason for the new call is that with PKS added behind kmap we have 3 levels
of mapping we want.

global kmap (can span threads and sleep)
'thread' kmap (can sleep but not span threads)
'atomic' kmap (can't sleep nor span threads [by definition])

As Matthew said perhaps 'global kmaps' may be best changed to vmaps?  I just
don't know the details of every call site.

And since I don't know the call site details if there are kmap_thread() calls
which are better off as kmap_atomic() calls I think it is worth converting
them.  But I made the assumption that kmap users would already be calling
kmap_atomic() if they could (because it is more efficient).

Ira