Re: [PATCH v2 0/4] hugetlbfs fallocate hole punch race with page faults

[Hugh Dickins]

On Wed, 28 Oct 2015, Mike Kravetz wrote:
> On 10/27/2015 08:34 PM, Hugh Dickins wrote:
> 
> Thanks for the detailed response Hugh.  I will try to address your questions
> and provide more reasoning behind the use case and need for this code.

And thank you for your detailed response, Mike: that helped a lot.

> Ok, here is a bit more explanation of the proposed use case.  It all
> revolves around a DB's use of hugetlbfs and the desire for more control
> over the underlying memory.  This additional control is achieved by
> adding existing fallocate and userfaultfd semantics to hugetlbfs.
> 
> In this use case there is a single process that manages hugetlbfs files
> and the underlying memory resources.  It pre-allocates/initializes these
> files.
> 
> In addition, there are many other processes which access (rw mode) these
> files.  They will simply mmap the files.  It is expected that they will
> not fault in any new pages.  Rather, all pages would have been pre-allocated
> by the management process.
> 
> At some time, the management process determines that specific ranges of
> pages within the hugetlbfs files are no longer needed.  It will then punch
> holes in the files.  These 'free' pages within the holes may then be used
> for other purposes.  For applications like this (sophisticated DBs), huge
> pages are reserved at system init time and closely managed by the
> application.
> Hence, the desire for this additional control.
> 
> So, when a hole containing N huge pages is punched, the management process
> wants to know that it really has N huge pages for other purposes.  Ideally,
> none of the other processes mapping this file/area would access the hole.
> This is an application error, and it can be 'caught' with  userfaultfd.
> 
> Since these other (non-management) processes will never fault in pages,
> they would simply set up userfaultfd to catch any page faults immediately
> after mmaping the hugetlbfs file.
> 
> > 
> > But it sounds to me more as if the holes you want punched are not
> > quite like on other filesystems, and you want to be able to police
> > them afterwards with userfaultfd, to prevent them from being refilled.
> 
> I am not sure if they are any different.
> 
> One could argue that a hole punch operation must always result in all
> pages within the hole being deallocated.  As you point out, this could
> race with a fault.  Previously, there would be no way to determine if
> all pages had been deallocated because user space could not detect this
> race.  Now, userfaultfd allows user space to catch page faults.  So,
> it is now possible to catch/depend on hole punch deallocating all pages
> within the hole.
> 
> > 
> > Can't userfaultfd be used just slightly earlier, to prevent them from
> > being filled while doing the holepunch?  Then no need for this patchset?
> 
> I do not think so, at least with current userfaultfd semantics.  The hole
> needs to be punched before being caught with UFFDIO_REGISTER_MODE_MISSING.

Great, that makes sense.

I was worried that you needed some kind of atomic treatment of the whole
extent punched, but all you need is to close the hole/fault race one
hugepage at a time.

Throw away all of 1/4, 2/4, 3/4: I think all you need is your 4/4
(plus i_mmap_lock_write around the hugetlb_vmdelete_list of course).

There you already do the single hugepage hugetlb_vmdelete_list()
under mutex_lock(&hugetlb_fault_mutex_table[hash]).

And it should come as no surprise that hugetlb_fault() does most
of its work under that same mutex.

So once remove_inode_hugepages() unlocks the mutex, that page is gone
from the file, and userfaultfd UFFDIO_REGISTER_MODE_MISSING will do
what you want, won't it?

I don't think "my" code buys you anything at all: you're not in danger of
shmem's starvation livelock issue, partly because remove_inode_hugepages()
uses the simple loop from start to end, and partly because hugetlb_fault()
already takes the serializing mutex (no equivalent in shmem_fault()).

Or am I dreaming?

Hugh
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