From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4ED943DB31F for ; Mon, 1 Jun 2026 15:46:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780328769; cv=none; b=iPv3Cy73nQYPtJIyIaOQtP+ydNfsiC3S+ZjtG90uaVe7qosUh45y5ks4mwqZZg5fhDsbuiUfWdgdLjURBH1nyZiMIVELZaWpeUf4WEqHaznFJ3dU5UX4YJbnRkmBAiMToFMCUzu+k7qvntRP0Jit3P5dyF1X4pMF5mjnOXM15oM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780328769; c=relaxed/simple; bh=e76JY67NsCDBswIqbEegTiA4v9dMTNGPsVWBdpfHC7I=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Rnj6IxSNvW7eNM6S6ZTY7ALZWrhUZMskcMs58gFNcqSyzaQTW5XD1VFaudEFNHqZzmpp2w2t9kxh0kSGDQ4WOinuwanAVEh+hUIPb164q0Z745DGEKTyMDSfmeIlgkTUd4KmO/GGHeqPYP1XtVB5wHBjLvsSl3IiER1d2ij2OdQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hoqPuJnq; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="hoqPuJnq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B58051F00893; Mon, 1 Jun 2026 15:46:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780328767; bh=wrlHsS4KRfDsGdr749kmn/DhUCPw/KOCalL8i8LYz1c=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=hoqPuJnq8gdBJUlYahWPLphzL2cdaRnmCaJJDfN6Np/bczfyC7ctpgnkj++HhD7/k Q6nf1As5sN2YFhs0WU9+DZ2JgOu3gWLtpJBZzsIf+03K8vYL5E8dntJs/CDglFb4sj Xaeio0WzbGqheiV7NylPwAUy+W8wQ+7C4SftFqvbv1wkD8HnaUO4RbtigDxGuZQWWK T8kKxeFQFh5ULjrWBt/luGuXTmAwedxY1p49K4OfjD78IfmbG/MM7e9iZvnJBV2d3X UUNKVpyycNPGyuo5L8AcuswU2aM8PIDvwzasjWWKitcgmkjAeuCkeGymyv5N/CHI5P +oOmpCtRMPuoQ== Date: Mon, 1 Jun 2026 16:46:02 +0100 From: Lorenzo Stoakes To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Arnd Bergmann , Greg Kroah-Hartman , "Liam R . Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jann Horn , Pedro Falcato , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v2 3/3] mm/vma: eliminate mmap_action->error_hook, introduce error_filter Message-ID: References: <88aa89e7-aa7e-4abf-babb-a2855bcb3fee@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <88aa89e7-aa7e-4abf-babb-a2855bcb3fee@kernel.org> On Mon, Jun 01, 2026 at 05:25:46PM +0200, David Hildenbrand (Arm) wrote: > On 5/22/26 18:00, Lorenzo Stoakes wrote: > > Rather than providing a hook, simplify things by providing the ability to > > filter errors. This allows us to more carefully validate the value provided > > and thus ensure only a valid error code is specified, and simplifies the > > interface. > > > > This way, we eliminate all hooks but mmap_prepare and allow only mmap > > actions to be specified (which core mm controls). > > > > This significantly improves robustness and eliminates any unnecessary code > > duplication in driver mmap hooks. > > > > We also update the /dev/mem logic (the only user) to use > > mmap_action->error_filter instead. > > > > Signed-off-by: Lorenzo Stoakes > > --- > > drivers/char/mem.c | 8 +------- > > include/linux/mm_types.h | 9 +++------ > > mm/util.c | 29 +++++++++++++++++++++-------- > > tools/testing/vma/include/dup.h | 9 +++------ > > 4 files changed, 28 insertions(+), 27 deletions(-) > > > > diff --git a/drivers/char/mem.c b/drivers/char/mem.c > > index a4297eb39887..11639d988e47 100644 > > --- a/drivers/char/mem.c > > +++ b/drivers/char/mem.c > > @@ -322,11 +322,6 @@ static const struct vm_operations_struct mmap_mem_ops = { > > #endif > > }; > > > > -static int mmap_filter_error(int err) > > -{ > > - return -EAGAIN; > > -} > > - > > static int mmap_mem_prepare(struct vm_area_desc *desc) > > { > > struct file *file = desc->file; > > @@ -362,8 +357,7 @@ static int mmap_mem_prepare(struct vm_area_desc *desc) > > > > /* Remap-pfn-range will mark the range with the I/O flag. */ > > mmap_action_remap_full(desc, desc->pgoff); > > - /* We filter remap errors to -EAGAIN. */ > > - desc->action.error_hook = mmap_filter_error; > > + desc->action.error_filter = -EAGAIN; > > > > return 0; > > } > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > > index 945c0a5386d6..8d1fb85e7684 100644 > > --- a/include/linux/mm_types.h > > +++ b/include/linux/mm_types.h > > @@ -844,13 +844,10 @@ struct mmap_action { > > enum mmap_action_type type; > > > > /* > > - * If specified, this hook is invoked when an error occurred when > > - * attempting the selected action. > > - * > > - * The hook can return an error code in order to filter the error, but > > - * it is not valid to clear the error here. > > + * If non-zero, filter errors that arise from mmap actions such that we > > + * return error_filter instead. Only valid error codes may be specified. > > Is that really a filter or rather an "error conversion" / "error override". > > "Filter" to my German brain implies that we would ... filter selected error > codes, not convert them to something else? I thik it's what people tend to call this, it's what I've seen it called anyway! I mean your German brain is being logical :) but by convention going with this terminology. > > > */ > > - int (*error_hook)(int err); > > + int error_filter; > > > > /* > > * This should be set in rare instances where the operation required > > diff --git a/mm/util.c b/mm/util.c > > index 4e172990afcd..9b4e5432d45a 100644 > > --- a/mm/util.c > > +++ b/mm/util.c > > @@ -1414,16 +1414,22 @@ static int mmap_action_finish(struct vm_area_struct *vma, > > */ > > len = vma_pages(vma) << PAGE_SHIFT; > > do_munmap(current->mm, vma->vm_start, len, NULL); > > - if (action->error_hook) { > > - /* We may want to filter the error. */ > > - err = action->error_hook(err); > > - /* The caller should not clear the error. */ > > - VM_WARN_ON_ONCE(!err); > > - } > > - return err; > > + > > + return action->error_filter ?: err; > > } > > Out of interest, why does dev/mem require this monstrosity? I'm not sure if it's really necessary, but I don't want to break userspace here by suddenly changing something that very plausible userspace might treat differently from other error codes (-EAGAIN). > > If it's really a dev/mem specialty, you could just make it less generic and call > the property > > "bool eagain_on_error;" > > because surely, we don't want any more such monstrosity? I'd rather not, because I suspect there might be some other cases of this, and it makes this option kinda horrible, as if it were a sane idea to convert errors to -EAGAIN :P Later on, once I've done more conversions, can obviously figure out a better way or whether we can just remove it. For now I want to eliminate these two horrible success/error hook hacks so we can move ahead more sanely (and I can in particular figure out a less horrible way of making the hugetlb 'vma lock' horror show work with mmap_prepare :) > > -- > Cheers, > > David Cheers, Lorenzo