Re: [aarcange@redhat.com: [PATCH] fork vs gup(-fast) fix]

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> Hi
> 
> following patch is my v2 approach.
> it survive Andrea's three dio test-case.
> 
> Linus suggested to change add_to_swap() and shrink_page_list() stuff
> for avoid false cow in do_wp_page() when page become to swapcache.
> 
> I think it's good idea. but it's a bit radical. so I think it's for development
> tree tackle.
> 
> Then, I decide to use Nick's early decow in 
> get_user_pages() and RO mapped page don't use gup_fast.
> 
> yeah, my approach is extream brutal way and big hammer. but I think 
> it don't have performance issue in real world.
> 
> why?
> 
> Practically, we can assume following two thing.
> 
> (1) the buffer of passed write(2) syscall argument is RW mapped
>     page or COWed RO page.
> 
> if anybody write following code, my path cause performance degression.
> 
>    buf = mmap()
>    memset(buf, 0x11, len);
>    mprotect(buf, len, PROT_READ)
>    fd = open(O_DIRECT)
>    write(fd, buf, len)
> 
> but it's very artifactical code. nobody want this.
> ok, we can ignore this.
> 
> (2) DirectIO user process isn't short lived process.
> 
> early decow only decrease short lived process performaqnce. 
> because long lived process do decowing anyway before exec(2).
> 
> and, All DB application is definitely long lived process.
> then early decow don't cause degression.

Frankly, linus sugessted to insert one branch into do_wp_page(), 
but I remove one branch from gup_fast.

I think it's good performance trade-off.
but if anybody hate my approach, I'll drop my chicken heart and
try to linus suggested way.





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