On Tue, Sep 03, 2024 at 07:04:51PM GMT, Suren Baghdasaryan wrote: > On Tue, Sep 3, 2024 at 6:17 PM Kent Overstreet > <kent.overstreet@xxxxxxxxx> wrote: > > > > On Tue, Sep 03, 2024 at 06:07:28PM GMT, Suren Baghdasaryan wrote: > > > On Sun, Sep 1, 2024 at 10:09 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > On Sun, 1 Sep 2024 21:41:27 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > > > > Introduce CONFIG_PGALLOC_TAG_REF_BITS to control the size of the > > > > > page allocation tag references. When the size is configured to be > > > > > less than a direct pointer, the tags are searched using an index > > > > > stored as the tag reference. > > > > > > > > > > ... > > > > > > > > > > +config PGALLOC_TAG_REF_BITS > > > > > + int "Number of bits for page allocation tag reference (10-64)" > > > > > + range 10 64 > > > > > + default "64" > > > > > + depends on MEM_ALLOC_PROFILING > > > > > + help > > > > > + Number of bits used to encode a page allocation tag reference. > > > > > + > > > > > + Smaller number results in less memory overhead but limits the number of > > > > > + allocations which can be tagged (including allocations from modules). > > > > > + > > > > > > > > In other words, "we have no idea what's best for you, you're on your > > > > own". > > > > > > > > I pity our poor users. > > > > > > > > Can we at least tell them what they should look at to determine whether > > > > whatever random number they chose was helpful or harmful? > > > > > > At the end of my reply in > > > https://lore.kernel.org/all/CAJuCfpGNYgx0GW4suHRzmxVH28RGRnFBvFC6WO+F8BD4HDqxXA@xxxxxxxxxxxxxx/#t > > > I suggested using all unused page flags. That would simplify things > > > for the user at the expense of potentially using more memory than we > > > need. > > > > Why would that use more memory, and how much? > > Say our kernel uses 5000 page allocations and there are additional 100 > allocations from all the modules we are loading at runtime. They all > can be addressed using 13 bits (8192 addressable tags), so the > contiguous memory we will be preallocating to store these tags is 8192 > * sizeof(alloc_tag). sizeof(alloc_tag) is 40 bytes as of today but > might increase in the future if we add more fields there for other > uses (like gfp_flags for example). So, currently this would use 320KB. > If we always use 16 bits we would be preallocating 2.5MB. So, that > would be 2.2MB of wasted memory. Using more than 16 bits (65536 > addressable tags) will be impractical anytime soon (current number > IIRC is a bit over 4000). I see, it's not about the page bits, it's about the contiguous array of alloc tags? What if we just reserved address space, and only filled it in as needed?
