Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP

From: Petr Mladek <pmladek@suse.com>
To: Luis Chamberlain <mcgrof@kernel.org>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
	"rppt@kernel.org" <rppt@kernel.org>,
	"songliubraving@fb.com" <songliubraving@fb.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"hch@infradead.org" <hch@infradead.org>,
	"ast@kernel.org" <ast@kernel.org>,
	"daniel@iogearbox.net" <daniel@iogearbox.net>,
	"Torvalds, Linus" <torvalds@linux-foundation.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"song@kernel.org" <song@kernel.org>,
	"Kernel-team@fb.com" <Kernel-team@fb.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"dborkman@redhat.com" <dborkman@redhat.com>,
	"edumazet@google.com" <edumazet@google.com>,
	"bp@alien8.de" <bp@alien8.de>, "mbenes@suse.cz" <mbenes@suse.cz>,
	"imbrenda@linux.ibm.com" <imbrenda@linux.ibm.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Adam Manzanares <a.manzanares@samsung.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>
Subject: Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP
Date: Wed, 20 Apr 2022 09:58:09 +0200	[thread overview]
Message-ID: <Yl+9ETR8qc1sRKy3@alley> (raw)
In-Reply-To: <Yl8olpqvZxY8KoNf@bombadil.infradead.org>

On Tue 2022-04-19 14:24:38, Luis Chamberlain wrote:
> On Tue, Apr 19, 2022 at 01:56:03AM +0000, Edgecombe, Rick P wrote:
> > Yea, that was my understanding. X86 modules have to be linked within
> > 2GB of the kernel text, also eBPF x86 JIT generates code that expects
> > to be within 2GB of the kernel text.
> 
> And kprobes / live patching / ftrace.
> 
> Another architectural fun fact, powerpc book3s/32 requires executability
> to be set per 256 Mbytes segments. Some architectures like this one
> will want to also optimize how they use the module alloc area.
> 
> Even though today the use cases might be limited, we don't exactly know
> how much memory a target device has a well, and so treating memory
> failures for "special memory" request as regular memory failures seems
> a bit odd, and users could get confused. For instance slapping on
> extra memory on a system won't resolve any issues if the limit for a
> special type of memory is already hit. Very likely not a problem at all today,
> given how small modules / eBPF jit programs are / etc, but conceptually it
> would seem wrong to just say -ENOMEM when in fact it's a special type of
> required memory which cannot be allocated and the issue cannot possibly be
> fixed. I don't think we have an option but to use -ENOMEM but at least
> hinting of the special failure would have seem desirable.

I am not mm expert but I think that this is common problem. There are
many types of "special memory". And mm provides many details via procfs, e.g.
/proc/meminfo, /proc/slabinfo, /proc/vmstat.

Best Regards,
Petr