On Tue 07-02-17 15:24:14, Vlastimil Babka wrote: > On 02/01/2017 03:05 PM, Michal Hocko wrote: > > From: Michal Hocko <mhocko@xxxxxxxx> > > > > __vmalloc* allows users to provide gfp flags for the underlying > > allocation. This API is quite popular > > $ git grep "=[[:space:]]__vmalloc\|return[[:space:]]*__vmalloc" | wc -l > > 77 > > > > the only problem is that many people are not aware that they really want > > to give __GFP_HIGHMEM along with other flags because there is really no > > reason to consume precious lowmemory on CONFIG_HIGHMEM systems for pages > > which are mapped to the kernel vmalloc space. About half of users don't > > use this flag, though. This signals that we make the API unnecessarily > > too complex. > > > > This patch simply uses __GFP_HIGHMEM implicitly when allocating pages to > > be mapped to the vmalloc space. Current users which add __GFP_HIGHMEM > > are simplified and drop the flag. > > > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > > --- > > Hi, > > this is based on top of [1]. I believe it was Al who has brought this > > up quite some time ago (or maybe I just misremember). The explicit > > usage of __GFP_HIGHMEM in __vmalloc* seems to be too much to ask from > > users. I believe there is no user which doesn't want vmalloc pages be > > in the highmem but I might be missing something. There is vmalloc_32* > > API but that uses GFP_DMA* explicitly which overrides __GFP_HIGHMEM. So > > all current users _should_ be safe to use __GFP_HIGHMEM unconditionally. > > This patch should simplify things and fix many users which consume > > lowmem for no good reason. > > > > I am sending this as an RFC to get some feedback, I even haven't compile > > tested it yet. > > > > Any comments are welcome. > > The idea sounds good. What are the potential dangers? That somebody of the > current callers without __GFP_HIGHMEM would take a physical address of the > page and then tried to access it via direct mapping? Yes, that wouldn't work but I do not think anybody would want to do something like that. Another risk would be that somebody really wanted to use vmalloc_32* but didn't use the proper API. The physically allocated page would then be used for a device which wouldn't be able to access it because it would be out of its addressable space. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
