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@xxxxxxxxx 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@xxxxxxxxxxxxx/
>
> 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
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