On Tue 17-01-17 21:59:13, John Hubbard wrote: > > On 01/16/2017 11:51 PM, Michal Hocko wrote: > > On Mon 16-01-17 13:57:43, John Hubbard wrote: > > > > > > > > > On 01/16/2017 01:48 PM, Michal Hocko wrote: > > > > On Mon 16-01-17 13:15:08, John Hubbard wrote: > > > > > > > > > > > > > > > On 01/16/2017 11:40 AM, Michal Hocko wrote: > > > > > > On Mon 16-01-17 11:09:37, John Hubbard wrote: > > > > > > > > > > > > > > > > > > > > > On 01/16/2017 12:47 AM, Michal Hocko wrote: > > > > > > > > On Sun 15-01-17 20:34:13, John Hubbard wrote: > > > > > > [...] > > > > > > > > > Is that "Reclaim modifiers" line still true, or is it a > > > > > > > > > leftover from an > > > > > > > > > earlier approach? I am having trouble reconciling it with > > > > > > > > > rest of the > > > > > > > > > patchset, because: > > > > > > > > > > > > > > > > > > a) the flags argument below is effectively passed on to > > > > > > > > > either kmalloc_node > > > > > > > > > (possibly adding, but not removing flags), or to > > > > > > > > > __vmalloc_node_flags. > > > > > > > > > > > > > > > > The above only says thos are _unsupported_ - in other words the > > > > > > > > behavior > > > > > > > > is not defined. Even if flags are passed down to kmalloc resp. > > > > > > > > vmalloc > > > > > > > > it doesn't mean they are used that way. Remember that vmalloc > > > > > > > > uses > > > > > > > > some hardcoded GFP_KERNEL allocations. So while I could be > > > > > > > > really > > > > > > > > strict about this and mask away these flags I doubt this is > > > > > > > > worth the > > > > > > > > additional code. > > > > > > > > > > > > > > I do wonder about passing those flags through to kmalloc. Maybe > > > > > > > it is worth > > > > > > > stripping out __GFP_NORETRY and __GFP_NOFAIL, after all. It > > > > > > > provides some > > > > > > > insulation from any future changes to the implementation of > > > > > > > kmalloc, and it > > > > > > > also makes the documentation more believable. > > > > > > > > > > > > I am not really convinced that we should take an extra steps for > > > > > > these > > > > > > flags. There are no existing users for those flags and new users > > > > > > should > > > > > > follow the documentation. > > > > > > > > > > OK, let's just fortify the documentation ever so slightly, then, so > > > > > that > > > > > users are more likely to do the right thing. How's this sound: > > > > > > > > > > * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not > > > > > supported. (Even > > > > > * though the current implementation passes the flags on through to > > > > > kmalloc and > > > > > * vmalloc, that is done for efficiency and to avoid unnecessary code. > > > > > The caller > > > > > * should not pass in these flags.) > > > > > * > > > > > * __GFP_REPEAT is supported, but only for large (>64kB) allocations. > > > > > > > > > > > > > > > ? Or is that documentation overkill? > > > > > > > > Dunno, it sounds like an overkill to me. It is telling more than > > > > necessary. If we want to be so vocal about gfp flags then we would have > > > > to say much more I suspect. E.g. what about __GFP_HIGHMEM? This flag is > > > > supported for vmalloc while unsupported for kmalloc. I am pretty sure > > > > there would be other gfp flags to consider and then this would grow > > > > borringly large and uninteresting to the point when people simply stop > > > > reading it. Let's just be as simple as possible. > > > > > > Agreed, on the simplicity point: simple and clear is ideal. But here, it's > > > merely short, and not quite simple. :) People will look at that short bit > > > of documentation, and then notice that the flags are, in fact, all passed > > > right on through down to both kmalloc_node and __vmalloc_node_flags. > > > > > > If you don't want too much documentation, then I'd be inclined to say > > > something higher-level, about the intent, rather than mentioning those two > > > flags directly. Because as it stands, the documentation contradicts what > > > the > > > code does. > > > > Feel free to suggest a better wording. I am, of course, open to any > > changes. > > OK, here's the best I've got, I tried to keep it concise, but (as you > suspected) I'm not sure it's actually any better than the original: > > * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL should not be passed in. > * Passing in __GFP_REPEAT is supported, but note that it is ignored for small > * (<=64KB) allocations, during the kmalloc attempt.
> __GFP_REPEAT is fully > * honored for all allocation sizes during the second part: the vmalloc > attempt. this is not true to be really precise because vmalloc doesn't respect the given gfp mask all the way down (look at the pte initialization). -- Michal Hocko SUSE Labs
