Messages in this thread | | | Subject | Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers | From | John Hubbard <> | Date | Mon, 16 Jan 2017 13:57:43 -0800 |
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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.
Sorry to go on and on about such a minor point. I'll let it go after this last note.
> -- > Michal Hocko > SUSE Labs >
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