[PATCH v2] makedumpfile: Exclude unnecessary hugepages.

[bhe at redhat.com]

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.