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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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <88aa89e7-aa7e-4abf-babb-a2855bcb3fee@kernel.org> X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: h5ecgx87qe7wu13rft4y95fo4b3hhoj8 X-Rspamd-Queue-Id: A74D4180011 X-HE-Tag: 1780328768-721183 X-HE-Meta: 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 AV1Bb+QT jmAb6YrzdgjxdyECM5pDdW8f+ICOZR5Nvqmx4XV4JggZjFt9r8ZP4VCTxUSRKUyUnqB6l4Y7jWFIW0AUOO3CARj1YevPFohU5DYnJEZEWrTZmzAHgEZEaV1L2fICEedZAXKwWY3AfLqzz2GYl2lDVH5xiM2gJanYMNmzqartLqjpnprSp+odU4/1SuJDt++1Xf+v8hm6OWQdO4/qjMqEVovztK3UxlYZdS0LA8aZdEwD978JEszbCraV3tAgw85ywcVyFLKYxHMhDeW5zjLExbnMry1QhCGLFcup6J7i32hOr3kl51ECaKCmPOJnGH10fge+y Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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