Messages in this thread | | | Date | Tue, 18 Jan 2011 16:41:33 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH v4] mm: add replace_page_cache_page() function |
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On Wed, 19 Jan 2011 09:27:33 +0900 Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> On Tue, 18 Jan 2011 15:28:44 -0800 > Andrew Morton <akpm@linux-foundation.org> wrote: > > > On Tue, 18 Jan 2011 12:18:11 +0100 > > Miklos Szeredi <miklos@szeredi.hu> wrote: > > > > > +int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask) > > > +{ > > > + int error; > > > + struct mem_cgroup *memcg = NULL; > > > > I'm suspecting that the unneeded initialisation was added to suppress a > > warning? > > > No. > It's necessary for mem_cgroup_{prepare|end}_migration(). > mem_cgroup_prepare_migration() will return without doing anything in > "if (mem_cgroup_disabled()" case(iow, "memcg" is not overwritten), > but mem_cgroup_end_migration() depends on the value of "memcg" to decide > whether prepare_migration has succeeded or not. > This may not be a good implementation,
It's a *surprising* implementation, in the context of kernel conventions. At least, it fooled me :)
Which makes it a not-good implementation, really. Also, it's undocumented.
> but IMHO I'd like to to initialize > valuable before using it in general.
mem_cgroup_prepare_migration() performs the initialisation! We do this in many places - it's the basic "function with more than one return value" mechanism.
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