linux-kernel - Re: ENOSPC returned by handle_mm

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Date:	Sun, 5 Jun 2011 12:16:08 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Al Viro <viro@...iv.linux.org.uk>
cc:	linux-mm@...ck.org, Mel Gorman <mel@....ul.ie>,
	linux-kernel@...r.kernel.org
Subject: Re: ENOSPC returned by handle_mm_fault()

On Sun, 5 Jun 2011, Al Viro wrote:
> 	When alloc_huge_page() runs afoul of quota, it returns ERR_PTR(-ENOSPC).
> Callers do not expect that - hugetlb_cow() returns ENOSPC if it gets that
> and so does hugetlb_no_page().  Eventually the thing propagates back to
> hugetlb_fault() and is returned by it.
> 
> 	Callers of hugetlb_fault() clearly expect a bitmap of VM_... and
> not something from errno.h: one place is 
>                         ret = hugetlb_fault(mm, vma, vaddr,
>                                 (flags & FOLL_WRITE) ? FAULT_FLAG_WRITE : 0);
>                         spin_lock(&mm->page_table_lock);
>                         if (!(ret & VM_FAULT_ERROR))
>                                 continue;
> and another is handle_mm_fault(), which ends up returning ENOSPC and *its*
> callers are definitely not ready to deal with that.
> 
> ENOSPC is 28, i.e. VM_FAULT_MAJOR | VM_FAULT_WRITE | VM_FAULT_HWPOISON;
> it's also theoretically possible to get ENOMEM if region_chg() ends up
> hitting
>                 nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
>                 if (!nrg)
>                         return -ENOMEM;
> region_chg() <- vma_needs_reservation() <- alloc_huge_page() and from that
> point as with ENOSPC.  ENOMEM is 12, i.e. VM_FAULT_MAJOR | VM_FAULT_WRITE...

Good find, news to me.  Interesting uses of -PTR_ERR()!
Looks like we'd better not have more than 12 VM_FAULT_ flags.

> 
> Am I right assuming that we want VM_FAULT_OOM in both cases?

No, where hugetlb_get_quota() fails it should be VM_FAULT_SIGBUS:
there's no excuse to go on an OOM-killing spree just because hugetlb
quota is exhausted.

VM_FAULT_OOM is appropriate where vma_needs_reservation() fails,
because region_chg() couldn't kmalloc a structure, as you point out.

(Though that doesn't matter much, since the only way the kmalloc can
fail is when this task is already selected for OOM-kill - I think.)

Hugh
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