On 07/18/14 at 06:08pm, Baoquan He wrote: > On 07/18/14 at 11:41am, Petr Tesarik wrote: > > On Fri, 18 Jul 2014 16:47:26 +0800 > > "bhe at redhat.com" <bhe at redhat.com> wrote: > > > > > On 07/18/14 at 09:38am, Petr Tesarik wrote: > > > > On Fri, 18 Jul 2014 10:16:16 +0800 > > > > "bhe at redhat.com" <bhe at redhat.com> wrote: > > > > > > > Thanks, Petr. This help. > > > > > > > I'm afraid you misunderstand the concept. Let me explain: > > > > > > > > page: > > > > The basic memory management block (usually 4K). > > > > > > > > compound page: > > > > The kernel may group adjacent pages into a larger object, and track > > > > page flags, refcount, etc. at one place for the whole group. This is > > > > called compound pages. > > > > > > > > Compound pages may be used for different things, either kernel > > > > internal allocations or by applications. > > > > > > > > Note that compound pages may or may not correspond to fewer levels of > > > > paging in hardware. Take x86_64 as an example. You can have compound > > > > pages with order=1 (i.e. 8K). There is nothing in the hardware to > > > > support such page size, so the page table just contains 2 consecutive > > > > 4K pages. Only if you have a compound page with order=9 (i.e. 2M), the > > > > kernel can use 3-level paging, creating a 2M page in hardware. > > > > > > > > hugetlbfs: > > > > Some compound pages are available through a special filesystem. This > > > > filesystem is used solely by user-space applications. However, these > > > > pages are formally owned by the kernel (after all, they belong to a > > > > filesystem, albeit a very special one). There is nothing in the page > > > > flags to tell that they are in fact used by user-space. > > > > > > I do have a confusion on this. Here it means user space benefits from > > > hugetlb only through hugetlbfs. If except of that, kernel may make use > > > of hugetlb to get large memory by merging continuous pages, doesn't it? > > > > Well, yes, it _is_ confusing: > > > > 1. hugetlbfs makes use of compound pages, but not every compound page > > is a hugetlbfs page, even if it is implemented with fewer levels of > > paging in hardware. > > I agree on this, it includes thp. And other kernel using from memory > allocation interface with __GFP_COMP flag. > > > > > 2. The kernel does not use hugetlbfs for its allocations. The kernel > > does use compound pages in many places. > > This confuses me. Could you please take several usage as example? > Because all metarial I can find says they are used for user space app. Yeah, you and Astushi are right. kmalloc will call kmalloc_large() if size exceeds a threshold. I need read those codes carefully.
