On Fri, Jan 20, 2023 at 11:15:17PM -0800, Ira Weiny wrote:
> Matthew Wilcox wrote:
> > I think I have a good folio replacement for memcpy_from_page(). One of
> > the annoying things about dealing with multi-page folios is that you
> > can't kmap the entire folio, yet on systems without highmem, you don't
> > need to. It's also somewhat annoying in the caller to keep track
> > of n/len/offset/pos/...
> >
> > I think this is probably the best option. We could have a loop that
> > kmaps each page in the folio, but that seems like excessive complexity.
>
> Why? IMO better to contain the complexity of highmem systems into any
> memcpy_[to,from]_folio() calls then spread them around the kernel.
Sure, but look at the conversion that I posted. It's actually simpler
than using the memcpy_from_page() API.
> > I'm happy to have highmem systems be less efficient, since they are
> > anyway. Another potential area of concern is that folios can be quite
> > large and maybe having preemption disabled while we copy 2MB of data
> > might be a bad thing. If so, the API is fine with limiting the amount
> > of data we copy, we just need to find out that it is a problem and
> > decide what the correct limit is, if it's not folio_size().
>
> Why not map the pages only when needed? I agree that keeping preemption
> disabled for a long time is a bad thing. But kmap_local_page does not
> disable preemption, only migration.
Some of the scheduler people aren't terribly happy about even disabling
migration for a long time. Is "copying 2MB of data" a long time? If I've
done my sums correctly, my current laptop has 2x 16 bit LP-DDR4-4267
DIMMs installed. That works out to 17GB/s and so copying 2MB of data
will take 118us. Probably OK for even the most demanding workload.
> Regardless any looping on the maps is going to only be on highmem systems
> and we can map the pages only if/when needed. Synchronization of the folio
> should be handled by the caller. So it is fine to all allow migration
> during memcpy_from_folio().
>
> So why not loop through the pages only when needed?
So you're proposing re-enabling migration after calling
kmap_local_folio()? I don't really understand.
> >
> > fs/ext4/verity.c | 16 +++++++---------
> > include/linux/highmem.h | 29 +++++++++++++++++++++++++++++
> > include/linux/page-flags.h | 1 +
> > 3 files changed, 37 insertions(+), 9 deletions(-)
> >
> > diff --git a/fs/ext4/verity.c b/fs/ext4/verity.c
> > index e4da1704438e..afe847c967a4 100644
> > --- a/fs/ext4/verity.c
> > +++ b/fs/ext4/verity.c
> > @@ -42,18 +42,16 @@ static int pagecache_read(struct inode *inode, void *buf, size_t count,
> > loff_t pos)
> > {
> > while (count) {
> > - size_t n = min_t(size_t, count,
> > - PAGE_SIZE - offset_in_page(pos));
> > - struct page *page;
> > + struct folio *folio;
> > + size_t n;
> >
> > - page = read_mapping_page(inode->i_mapping, pos >> PAGE_SHIFT,
> > + folio = read_mapping_folio(inode->i_mapping, pos >> PAGE_SHIFT,
> > NULL);
>
> Is this an issue with how many pages get read into the page
> cache? I went off on a tangent thinking this read the entire folio into
> the cache. But I see now I was wrong. If this is operating page by page
> why change this function at all?
The folio may (indeed _should_) be already present in the cache, otherwise
the cache isn't doing a very good job. If read_mapping_folio() ends up
having to allocate the folio, today it only allocates a single page folio.
But if somebody else allocated it through the readahead code, and the
filesystem supports multi-page folios, then it will be larger than a
single page. All callers must be prepared to handle a multi-page folio.
> > - if (IS_ERR(page))
> > - return PTR_ERR(page);
> > -
> > - memcpy_from_page(buf, page, offset_in_page(pos), n);
> > + if (IS_ERR(folio))
> > + return PTR_ERR(folio);
> >
> > - put_page(page);
> > + n = memcpy_from_file_folio(buf, folio, pos, count);
> > + folio_put(folio);
> >
> > buf += n;
> > pos += n;
> > diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> > index 9fa462561e05..9917357b9e8f 100644
> > --- a/include/linux/highmem.h
> > +++ b/include/linux/highmem.h
> > @@ -414,6 +414,35 @@ static inline void memzero_page(struct page *page, size_t offset, size_t len)
> > kunmap_local(addr);
> > }
> >
> > +/**
> > + * memcpy_from_file_folio - Copy some bytes from a file folio.
> > + * @to: The destination buffer.
> > + * @folio: The folio to copy from.
> > + * @pos: The position in the file.
> > + * @len: The maximum number of bytes to copy.
> > + *
> > + * Copy up to @len bytes from this folio. This may be limited by PAGE_SIZE
>
> I have a problem with 'may be limited'. How is the caller to know this?
... from the return value?
> Won't this propagate a lot of checks in the caller? Effectively replacing
> one complexity in the callers for another?
Look at the caller I converted! It _reduces_ the amount of checks in
the caller.
> > + * if the folio comes from HIGHMEM, and by the size of the folio.
> > + *
> > + * Return: The number of bytes copied from the folio.
> > + */
> > +static inline size_t memcpy_from_file_folio(char *to, struct folio *folio,
> > + loff_t pos, size_t len)
> > +{
> > + size_t offset = offset_in_folio(folio, pos);
> > + char *from = kmap_local_folio(folio, offset);
> > +
> > + if (folio_test_highmem(folio))
> > + len = min(len, PAGE_SIZE - offset);
> > + else
> > + len = min(len, folio_size(folio) - offset);
> > +
> > + memcpy(to, from, len);
>
> Do we need flush_dcache_page() for the pages?
Why? memcpy_from_page() doesn't have one.
> I gave this an attempt today before I realized read_mapping_folio() only
> reads a single page. :-(
>
> How does memcpy_from_file_folio() work beyond a single page? And in that
> case what is the point? The more I think about this the more confused I
> get.
In the highmem case, we map a single page and so cannot go beyond a
single page. If the folio wasn't allocated from highmem, we're just
using its address directly, so we can access the whole folio.
Hope I've cleared up your confusions.
