[CCing linux-mm] On Tue 22-12-15 21:04:35, Al Viro wrote: [...] > Documentation/which-allocator-should-I-use might be a good idea... Notes > below are just a skeleton - a lot of details need to be added; in particular, > there should be a part on "I have this kind of address and I want that; > when and how should that be done?", completely missing here. And there > should be a big scary warning along the lines of "this is NOT an invitation > for a flood of checkpatch-inspired patches"... > > Comments, corrections and additions would be very welcome. FWIW I think this is a very good idea. The current form is good enough IMHO. > 1) Most of the time kmalloc() is the right thing to use. > Limitations: alignment is no better than word, not available very early in > bootstrap, allocated memory is physically contiguous, so large allocations > are best avoided. > > 2) kmem_cache_alloc() allows to specify the alignment at cache creation > time. Otherwise it's similar to kmalloc(). Normally it's used for > situations where we have a lot of instances of some type and want dynamic > allocation of those. > > 3) vmalloc() is for large allocations. They will be page-aligned, > but *not* physically contiguous. OTOH, large physically contiguous > allocations are generally a bad idea. Unlike other allocators, there's > no variant that could be used in interrupt; freeing is possible there, > but allocation is not. Note that non-blocking variant *does* exist - > __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL) can be used in atomic > contexts; it's the interrupt ones that are no-go. It is also hardcoded GFP_KERNEL context so a usage from NOFS context needs a special treatment. > 4) if it's very early in bootstrap, alloc_bootmem() and friends > may be the only option. Rule of the thumb: if it's already printed > Memory: ...../..... available..... > you shouldn't be using that one. Allocations are physically contiguous > and at that point large physically contiguous allocations are still OK. > > 5) if you need to allocate memory for DMA, use dma_alloc_coherent() > and friends. They'll give you both the virtual address for your use > and DMA address refering to the same memory for use by device; do *NOT* > try to derive the latter from the former; use of virt_to_bus() et.al. > is a Bloody Bad Idea(tm). > > 6) if you need a reference to struct page, use alloc_page/alloc_pages. > > 7) in some cases (page tables, for the most obvious example), __get_free_page() > and friends might be the right answer. In principle, it's case (6), but > it returns page_address(page) instead of the page itself. Historically that > was the first API introduced, so a _lot_ of places that should've been using > something else ended up using that. Do not assume that being lower level > makes it faster than e.g. kmalloc() - this is simply not true. -- 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>