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 dKkg72hF vN5z/jBseTnHSu9tf89bfZNrlSJYNnkSRGEqWFu6WeJqFt1z1S9TF6QMbZjOG5ImXIs1tsDlDFzBe+3mywVTsBO7XL3b/5s4fJnvhSzHJUNu9fiLj53pu5DwXBKdSUBFDkjnYdetmhcP7YGPk0r/EtCptYJfAUpforziXPGVL7w9fVPNxbOnHjss4G2l5hlCjBom8o8lKzkgozToETKE5r1Hl2ovJB5XaBIIczxOE2zPaOr59YiuXCIQOvxVudRRV9NXPLX8mXTa7nyf6pDprO8SqH9pnI9/4ikC1Fg6+l30i7QTFveSOF9L01IbFq/NI2BSsTsRzuCz3LHxMGvuvunik3BZAAvVDQVpiO2Yj/3nNTjIXyJ/7+cQj1ZzyYACtFYFX0Mwr9+jxjHBn+XZnlNIFWe0L2k3vOqcJ2BEKDYhiv36ZJ4rE8E7u1QzLVwHztsmzywBGN1VKaRWMQUT7j4MTKYmoqf1nFFHn X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> I just thought of named anonymous VMA may help. We can give the private >> /dev/zero mapping a name, for example, just "/dev/zero". However, >> "[anon:/dev/zero]" will show up in smaps/maps. We can't keep the device >> numbers and inode number either, but it seems it can tell the user this >> mapping comes from /dev/zero, and it also explicitly tells us it is >> specially treated by kernel. Hopefully setting anon_name is permitted. > But then that'd require CONFIG_ANON_VMA_NAME unfortunately :( Yes. > > I think this maps thing is the killer here really. > > It'd be nice to -specifically- have a means of expressing this kind of VMA, > we have a means of setting a VMA anon, so maybe we can 'set a VMA to > /dev/zero' and somehow explicitly know that we've done this and identify > this special case. > > I'm not sure that the .mmap callback is the right place to do this and I"m > not sure how exactly this would work but this could be workable. A couple of potential approaches off the top of my head:   - A new vm flag   - Use vm_private_data Both of them have pros and cons. The vm flag is simple enough, but it needs to consume one bit for just one usecase. The vm_private_data is a void pointer and a lot drivers use it to store driver specific data structures, so using the pointer in a generic path (for example, smaps) to tell us whether it is /dev/zero is not easy. We may be able to have a special encoding to it, for example, set the last bit (the trick is not unusual in core mm code). > > I agree the actual offset into the zero page is of no relevance and no > _sane_ user will care, but this way we could put /dev/zero in [s]maps, > treat this VMA as anon, but also add semantic information about the > existence of this weird corner case. >