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SubjectRe: [RFC PATCH V2] iomap: add support to track dirty state of sub pages
On Tue, Aug 18, 2020 at 04:53:05PM +0100, Matthew Wilcox wrote:
> On Tue, Aug 18, 2020 at 09:46:18PM +0800, Yu Kuai wrote:
> > changes from v1:
> > - separate set dirty and clear dirty functions
> > - don't test uptodate bit in iomap_writepage_map()
> > - use one bitmap array for uptodate and dirty.
>
> This looks much better.
>
> > + spinlock_t state_lock;
> > + /*
> > + * The first half bits are used to track sub-page uptodate status,
> > + * the second half bits are for dirty status.
> > + */
> > + DECLARE_BITMAP(state, PAGE_SIZE / 256);
>
> It would be better to use the same wording as below:
>
> > + bitmap_zero(iop->state, PAGE_SIZE * 2 / SECTOR_SIZE);

ISTR there was some reason why '512' was hardcoded in here instead of
SECTOR_SIZE. I /think/ it was so that iomap.h did not then have a hard
dependency on blkdev.h and everything else that requires...

https://lore.kernel.org/linux-xfs/20181215105155.GD1575@lst.de/

--D

>
> [...]
>
> > +static void
> > +iomap_iop_set_range_dirty(struct page *page, unsigned int off,
> > + unsigned int len)
> > +{
> > + struct iomap_page *iop = to_iomap_page(page);
> > + struct inode *inode = page->mapping->host;
> > + unsigned int total = PAGE_SIZE / SECTOR_SIZE;
> > + unsigned int first = off >> inode->i_blkbits;
> > + unsigned int last = (off + len - 1) >> inode->i_blkbits;
> > + unsigned long flags;
> > + unsigned int i;
> > +
> > + spin_lock_irqsave(&iop->state_lock, flags);
> > + for (i = first; i <= last; i++)
> > + set_bit(i + total, iop->state);
> > + spin_unlock_irqrestore(&iop->state_lock, flags);
> > +}
>
> How about:
>
> - unsigned int total = PAGE_SIZE / SECTOR_SIZE;
> ...
> + first += PAGE_SIZE / SECTOR_SIZE;
> + last += PAGE_SIZE / SECTOR_SIZE;
> ...
> for (i = first; i <= last; i++)
> - set_bit(i + total, iop->state);
> + set_bit(i, iop->state);
>
> We might want
>
> #define DIRTY_BITS(x) ((x) + PAGE_SIZE / SECTOR_SIZE)
>
> and then we could do:
>
> + unsigned int last = DIRTY_BITS((off + len - 1) >> inode->i_blkbits);
>
> That might be overthinking things a bit though.
>
> > @@ -705,6 +767,7 @@ __iomap_write_end(struct inode *inode, loff_t pos, unsigned len,
> > if (unlikely(copied < len && !PageUptodate(page)))
> > return 0;
> > iomap_set_range_uptodate(page, offset_in_page(pos), len);
> > + iomap_set_range_dirty(page, offset_in_page(pos), len);
> > iomap_set_page_dirty(page);
>
> I would move the call to iomap_set_page_dirty() into
> iomap_set_range_dirty() to parallel iomap_set_range_uptodate more closely.
> We don't want a future change to add a call to iomap_set_range_dirty()
> and miss the call to iomap_set_page_dirty().
>
> > return copied;
> > }
> > @@ -1030,6 +1093,7 @@ iomap_page_mkwrite_actor(struct inode *inode, loff_t pos, loff_t length,
> > WARN_ON_ONCE(!PageUptodate(page));
> > iomap_page_create(inode, page);
> > set_page_dirty(page);
> > + iomap_set_range_dirty(page, offset_in_page(pos), length);
>
> I would move all this from the mkwrite_actor() to iomap_page_mkwrite()
> and call it once with (0, PAGE_SIZE) rather than calling it once for
> each extent in the page.
>
> > @@ -1435,6 +1500,8 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> > */
> > set_page_writeback_keepwrite(page);
> > } else {
> > + iomap_clear_range_dirty(page, 0,
> > + end_offset - page_offset(page) + 1);
> > clear_page_dirty_for_io(page);
> > set_page_writeback(page);
>
> I'm not sure it's worth doing this calculation. Better to just clear
> the dirty bits on the entire page? Opinions?

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