On Wed, 21 Mar 2018, Christopher Lameter wrote: > On Wed, 21 Mar 2018, Mikulas Patocka wrote: > > > > You should not be using the slab allocators for these. Allocate higher > > > order pages or numbers of consecutive smaller pagess from the page > > > allocator. The slab allocators are written for objects smaller than page > > > size. > > > > So, do you argue that I need to write my own slab cache functionality > > instead of using the existing slab code? > > Just use the existing page allocator calls to allocate and free the > memory you need. > > > I can do it - but duplicating code is bad thing. > > There is no need to duplicate anything. There is lots of infrastructure > already in the kernel. You just need to use the right allocation / freeing > calls. So, what would you recommend for allocating 640KB objects while minimizing wasted space? * alloc_pages - rounds up to the next power of two * kmalloc - rounds up to the next power of two * alloc_pages_exact - O(n*log n) complexity; and causes memory fragmentation if used excesivelly * vmalloc - horrible performance (modifies page tables and that causes synchronization across all CPUs) anything else? The slab cache with large order seems as a best choice for this. > > > What kind of problem could be caused here? > > > > Unlocked accesses are generally considered bad. For example, see this > > piece of code in calculate_sizes: > > s->allocflags = 0; > > if (order) > > s->allocflags |= __GFP_COMP; > > > > if (s->flags & SLAB_CACHE_DMA) > > s->allocflags |= GFP_DMA; > > > > if (s->flags & SLAB_RECLAIM_ACCOUNT) > > s->allocflags |= __GFP_RECLAIMABLE; > > > > If you are running this while the cache is in use (i.e. when the user > > writes /sys/kernel/slab/<cache>/order), then other processes will see > > invalid s->allocflags for a short time. > > Calculating sizes is done when the slab has only a single accessor. Thus > no locking is neeed. The calculation is done whenever someone writes to "/sys/kernel/slab/*/order" And you can obviously write to that file why the slab cache is in use. Try it. So, the function calculate_sizes can actually race with allocation from the slab cache. > Changing the size of objects in a slab cache when there is already a set > of object allocated and under management by the slab cache would > cause the allocator to fail and lead to garbled data. I am not talking about changing the size of objects in a slab cache. I am talking about changing the allocation order of a slab cache while the cache is in use. This can be done with the sysfs interface. Mikulas
