Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped_alloc()

[Michal Hocko]

On Thu 30-03-23 08:13:48, Mike Rapoport wrote:
> On Wed, Mar 29, 2023 at 10:13:23AM +0200, Michal Hocko wrote:
> > On Wed 29-03-23 10:28:02, Mike Rapoport wrote:
> > > On Tue, Mar 28, 2023 at 05:24:49PM +0200, Michal Hocko wrote:
> > > > On Tue 28-03-23 18:11:20, Mike Rapoport wrote:
> > > > > On Tue, Mar 28, 2023 at 09:39:37AM +0200, Michal Hocko wrote:
> > > > [...]
> > > > > > OK, so you want to reduce that direct map fragmentation?
> > > > > 
> > > > > Yes.
> > > > > 
> > > > > > Is that a real problem?
> > > > > 
> > > > > A while ago Intel folks published report [1] that showed better performance
> > > > > with large pages in the direct map for majority of benchmarks.
> > > > > 
> > > > > > My impression is that modules are mostly static thing. BPF
> > > > > > might be a different thing though. I have a recollection that BPF guys
> > > > > > were dealing with direct map fragmention as well.
> > > > > 
> > > > > Modules are indeed static, but module_alloc() used by anything that
> > > > > allocates code pages, e.g. kprobes, ftrace and BPF. Besides, Thomas
> > > > > mentioned that having code in 2M pages reduces iTLB pressure [2], but
> > > > > that's not only about avoiding the splits in the direct map but also about
> > > > > using large mappings in the modules address space.
> > > > > 
> > > > > BPF guys suggested an allocator for executable memory [3] mainly because
> > > > > they've seen performance improvement of 0.6% - 0.9% in their setups [4].
> > > > 
> > > > These are fair arguments and it would have been better to have them in
> > > > the RFC. Also it is not really clear to me what is the actual benefit of
> > > > the unmapping for those usecases. I do get they want to benefit from
> > > > caching on the same permission setup but do they need unmapping as well?
> > > 
> > > The pages allocated with module_alloc() get different permissions depending
> > > on whether they belong to text, rodata, data etc. The permissions are
> > > updated in both vmalloc address space and in the direct map. The updates to
> > > the direct map cause splits of the large pages.
> > 
> > That much is clear (wouldn't hurt to mention that in the changelog
> > though).
> > 
> > > If we cache large pages as
> > > unmapped we take out the entire 2M page from the direct map and then
> > > if/when it becomes free it can be returned to the direct map as a 2M page.
> > > 
> > > Generally, the unmapped allocations are intended for use-cases that anyway
> > > map the memory elsewhere than direct map and need to modify direct mappings
> > > of the memory, be it modules_alloc(), secretmem, PKS page tables or maybe
> > > even some of the encrypted VM memory.
> > 
> > I believe we are still not on the same page here. I do understand that
> > you want to re-use the caching capability of the unmapped_pages_alloc
> > for modules allocations as well. My question is whether module_alloc
> > really benefits from the fact that the memory is unmapped?
> > 
> > I guess you want to say that it does because it wouldn't have to change
> > the permission for the direct map but I do not see that anywhere in the
> > patch...
> 
> This happens automagically outside the patch :)
> 
> Currently, to change memory permissions modules code calls set_memory APIs
> and passes vmalloced address to them. set_memory functions lookup the
> direct map alias and update the permissions there as well.
> If the memory allocated with module_alloc() is unmapped in the direct map,
> there won't be an alias for set_memory APIs to update.
> 
> > Also consinder the slow path where module_alloc needs to
> > allocate a fresh (huge)page and unmap it. Does the overhead of the
> > unmapping suits needs of all module_alloc users? Module loader is likely
> > not interesting as it tends to be rather static but what about BPF
> > programs?
> 
> The overhead of unmapping pages in the direct map on allocation path will
> be offset by reduced overhead of updating permissions in the direct map
> after the allocation. Both are using the same APIs and if today the
> permission update causes a split of a large page, unmapping of a large page
> won't.
> 
> Of course in a loaded system unmapped_alloc() won't be able to always
> allocated large pages to replenish the cache, but still there will be fewer
> updates to the direct map.
 
Ok, all of that is a changelog material. I would recommend to go this
way. Start by the simple unmapped_pages_alloc interface and use it for
the secret memory. There shouldn't be anything controversial there. In a
follow up patch add a support for the vmalloc which would add a new gfp
flag with a justification that this is the simplest way to support
module_alloc usecase and do not feel shy of providing as much context as
you can. Ideally with some numbers for the best/worst/avg cases.

Thanks

-- 
Michal Hocko
SUSE Labs