linux-kernel - Re: [PATCH v16 1/5] mm: add VM_DROPPABLE for designating always lazily freeable mappings

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Date: Fri, 31 May 2024 12:48:58 +0200
From: Jann Horn <jannh@...gle.com>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: linux-kernel@...r.kernel.org, patches@...ts.linux.dev, tglx@...utronix.de, 
	linux-crypto@...r.kernel.org, linux-api@...r.kernel.org, x86@...nel.org, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>, "Carlos O'Donell" <carlos@...hat.com>, 
	Florian Weimer <fweimer@...hat.com>, Arnd Bergmann <arnd@...db.de>, 
	Christian Brauner <brauner@...nel.org>, David Hildenbrand <dhildenb@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH v16 1/5] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings

On Tue, May 28, 2024 at 2:24 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
> c) If there's not enough memory to service a page fault, it's not fatal.
[...]
> @@ -5689,6 +5689,10 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>
>         lru_gen_exit_fault();
>
> +       /* If the mapping is droppable, then errors due to OOM aren't fatal. */
> +       if (vma->vm_flags & VM_DROPPABLE)
> +               ret &= ~VM_FAULT_OOM;

Can you remind me how this is supposed to work? If we get an OOM
error, and the error is not fatal, does that mean we'll just keep
hitting the same fault handler over and over again (until we happen to
have memory available again I guess)?

Or is there something in this series that somehow redirects userspace
execution to getrandom() in that case?


> +
>         if (flags & FAULT_FLAG_USER) {
>                 mem_cgroup_exit_user_fault();
>                 /*