Re: [PATCH] mm: implement POSIX_FADV_NOREUSE

[Lukas Senger <lukas@fridolin.com>]

> But, why wouldn't this work there?  Define a percpu variable, and assign
> it to the target page in readahead's read_pages() and in
> do_generic_file_read() which deal with pages one at a time and not in lists.
> 
> struct page *read_me_once;
> void hint_page_read_once(struct page *page)
> {
> 	read_me_once = page;
> }
> 
> Then check for (read_me_once == page) in add_page_to_lru_list() instead
> of the page flag.  Then, make read_me_once per-cpu.  This won't be
> preempt safe, but we're talking about readahead and hints here, so we
> can probably just bail in the cases where we race.

Thanks for clarifying that. The problem now is that by the time we get
to add_page_to_lru_list we're dealing with multiple pages again, because
of the buffering in pagevecs. We could do the (read_me_once == page)
check in __lru_cache_add and then add it to a (new) lru_add_tail_pvec
that adds its pages to the tail of the lru_lists.

If this way isn't feasible, we'll take a look at Andrew and Michal's
DONTNEED lite idea. However, with a DONTNEED lite implemented in the
posix_fadvise, the syscall would be more cumbersome to use for
application programmers. They would need to call it after every read.
The tail-pvec approach only needs a single syscall after open, as do
NORMAL, SEQUENTIAL and RANDOM. Furthermore we suspect that implementing
it in a way that respects other processes (unlike DONTNEED) won't be
much simpler than the tail-pvec approach.

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