On Wed 12-02-20 14:16:14, Minchan Kim wrote:
> PG_readahead flag is shared with PG_reclaim but PG_reclaim is only
> used in write context while PG_readahead is used for read context.
>
> To make it clear, let's introduce PageReadahead wrapper with
> !PageWriteback so it could make code clear and we could drop
> PageWriteback check in page_cache_async_readahead, which removes
> pointless dropping mmap_sem.
>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
...
> +/* Clear PG_readahead only if it's PG_readahead, not PG_reclaim */
> +static inline int TestClearPageReadahead(struct page *page)
> +{
> + VM_BUG_ON_PGFLAGS(PageCompound(page), page);
> +
> + return !PageWriteback(page) ||
> + test_and_clear_bit(PG_reclaim, &page->flags);
> +}
I think this is still wrong - if PageWriteback is not set, it will never
clear PG_reclaim bit so effectively the page will stay in PageReadahead
state!
The logic you really want to implement is:
if (PageReadahead(page)) { <- this is your new PageReadahead
implementation
clear_bit(PG_reclaim, &page->flags);
return 1;
}
return 0;
Now this has the problem that it is not atomic. The only way I see to make
this fully atomic is using cmpxchg(). If we wanted to make this kinda-sorta
OK, the proper condition would look like:
return !PageWriteback(page) **&&**
test_and_clear_bit(PG_reclaim, &page->flags);
Which is similar to what you originally had but different because in C '&&'
operator is not commutative due to side-effects committed at sequence points.
BTW: I share Andrew's view that we are piling hacks to fix problems caused
by older hacks. But I don't see any good option how to unalias
PG_readahead and PG_reclaim.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
