On Sun 07-01-18 13:58:42, Dan Williams wrote:
> On Thu, Jan 4, 2018 at 3:12 AM, Jan Kara <jack@xxxxxxx> wrote:
> > On Sat 23-12-17 16:57:31, Dan Williams wrote:
> >
> >> + /*
> >> + * Flush dax_dma_lock() sections to ensure all possible page
> >> + * references have been taken, or will block on the fs
> >> + * 'mmap_lock'.
> >> + */
> >> + synchronize_rcu();
> >
> > Frankly, I don't like synchronize_rcu() in a relatively hot path like this.
> > Cannot we just abuse get_dev_pagemap() to fail if truncation is in progress
> > for the pfn? We could indicate that by some bit in struct page or something
> > like that.
>
> We would need a lockless way to take a reference conditionally if the
> page is not subject to truncation.
>
> I recall the raid5 code did something similar where it split a
> reference count into 2 fields. I.e. take page->_refcount and use the
> upper bits as a truncation count. Something like:
>
> do {
> old = atomic_read(&page->_refcount);
> if (old & trunc_mask) /* upper bits of _refcount */
> return false;
> new = cnt + 1;
> } while (atomic_cmpxchg(&page->_refcount, old, new) != old);
> return true; /* we incremented the _refcount while the truncation
> count was zero */
>
> ...the only concern is teaching the put_page() path to consider that
> 'trunc_mask' when determining that the page is idle.
>
> Other ideas?
What I rather thought about was an update to GUP paths (like
follow_page_pte()):
if (flags & FOLL_GET) {
get_page(page);
if (pte_devmap(pte)) {
/*
* Pairs with the barrier in the truncate path.
* Could be possibly _after_atomic version of the
* barrier.
*/
smp_mb();
if (PageTruncateInProgress(page)) {
put_page(page);
..bail...
}
}
}
and in the truncate path:
down_write(inode->i_mmap_sem);
walk all pages in the mapping and mark them PageTruncateInProgress().
unmap_mapping_range(...);
/*
* Pairs with the barrier in GUP path. In fact not necessary since
* unmap_mapping_range() provides us with the barrier already.
*/
smp_mb();
/*
* By now we are either guaranteed to see grabbed page reference or
* GUP is guaranteed to see PageTruncateInProgress().
*/
while ((page = dax_find_referenced_page(mapping))) {
...
}
The barriers need some verification, I've opted for the conservative option
but I guess you get the idea.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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