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Date:   Wed, 20 Jan 2021 19:04:39 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Hildenbrand <david@redhat.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Mark Rutland <mark.rutland@arm.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
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        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Roman Gushchin <guro@fb.com>,
        Shakeel Butt <shakeelb@google.com>,
        Shuah Khan <shuah@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tycho Andersen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
        linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
        x86@kernel.org, Hagen Paul Pfeifer <hagen@jauu.net>
Subject: Re: [PATCH v14 05/10] mm: introduce memfd_secret system call to
 create "secret" memory areas
Message-ID: <20210120170439.GP1106298@kernel.org>
References: <20201203062949.5484-1-rppt@kernel.org>
 <20201203062949.5484-6-rppt@kernel.org>
 <20210119202213.GI2260413@casper.infradead.org>
 <20210120150510.GO1106298@kernel.org>
 <20210120160210.GK2260413@casper.infradead.org>
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On Wed, Jan 20, 2021 at 04:02:10PM +0000, Matthew Wilcox wrote:
> On Wed, Jan 20, 2021 at 05:05:10PM +0200, Mike Rapoport wrote:
> > On Tue, Jan 19, 2021 at 08:22:13PM +0000, Matthew Wilcox wrote:
> > > On Thu, Dec 03, 2020 at 08:29:44AM +0200, Mike Rapoport wrote:
> > > > +static vm_fault_t secretmem_fault(struct vm_fault *vmf)
> > > > +{
> > > > +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> > > > +	struct inode *inode = file_inode(vmf->vma->vm_file);
> > > > +	pgoff_t offset = vmf->pgoff;
> > > > +	vm_fault_t ret = 0;
> > > > +	unsigned long addr;
> > > > +	struct page *page;
> > > > +	int err;
> > > > +
> > > > +	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> > > > +		return vmf_error(-EINVAL);
> > > > +
> > > > +	page = find_get_page(mapping, offset);
> > > > +	if (!page) {
> > > > +
> > > > +		page = secretmem_alloc_page(vmf->gfp_mask);
> > > > +		if (!page)
> > > > +			return vmf_error(-ENOMEM);
> > > 
> > > Just use VM_FAULT_OOM directly.
> >  
> > Ok.
> > 
> > > > +		err = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);
> > > > +		if (unlikely(err))
> > > > +			goto err_put_page;
> > > 
> > > What if the error is EEXIST because somebody else raced with you to add
> > > a new page to the page cache?
> > 
> > Right, for -EEXIST I need a retry here, thanks.
> > 
> > > > +		err = set_direct_map_invalid_noflush(page, 1);
> > > > +		if (err)
> > > > +			goto err_del_page_cache;
> > > 
> > > Does this work correctly if somebody else has a reference to the page
> > > in the meantime?
> > 
> > Yes, it does. If somebody else won the race that page was dropped from the
> > direct map and this call would be essentially a nop. And anyway, the very
> > next patch changes the way pages are removed from the direct map ;-)
> 
> What I'm thinking is:
> 
> thread A page faults
> doesn't find page
> allocates page
> adds page to page cache
> 				thread B page faults
> 				does find page in page cache
> set direct map invalid fails
> deletes from page cache
> 				... ?

Hmm, this is not nice indeed...

-- 
Sincerely yours,
Mike.