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Date: Thu, 17 Nov 2022 15:15:40 -0800 From: John Hubbard <jhubbard@...dia.com> To: David Howells <dhowells@...hat.com>, Al Viro <viro@...iv.linux.org.uk> CC: Matthew Wilcox <willy@...radead.org>, <linux-mm@...ck.org>, <linux-fsdevel@...r.kernel.org>, Christoph Hellwig <hch@...radead.org>, "Jeff Layton" <jlayton@...nel.org>, <linux-kernel@...r.kernel.org> Subject: Re: [RFC PATCH 1/4] mm: Move FOLL_* defs to mm_types.h On 11/17/22 06:54, David Howells wrote: > Move FOLL_* definitions to linux/mm_types.h to make them more accessible > without having to drag in all of linux/mm.h and everything that drags in > too[1]. > > Suggested-by: Matthew Wilcox <willy@...radead.org> > Signed-off-by: David Howells <dhowells@...hat.com> > cc: John Hubbard <jhubbard@...dia.com> > cc: Al Viro <viro@...iv.linux.org.uk> > cc: linux-mm@...ck.org > cc: linux-fsdevel@...r.kernel.org > Link: https://lore.kernel.org/linux-fsdevel/Y1%2FhSO+7kAJhGShG@casper.infradead.org/ [1] > --- > > include/linux/mm.h | 74 ---------------------------------------------- > include/linux/mm_types.h | 73 +++++++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 73 insertions(+), 74 deletions(-) OK, I've verified that this is a "mostly identical" movement: the only thing that changes is that the comments now come before the defines. And because mm.h includes mm_types.h, it is unlikely that moving a define from mm.h to mm_types.h would cause build failures. It's not completely impossible: ordering issues are sometimes involved in this sort of change. But unlikely. Anyway, this is a good move. The users of various mm APIs should not have to pull in quite so much of the internals of mm, and this is a step in that direction. FOLL_* items are used by filesystems and other subsystems that definitely do not need all of mm.h. Reviewed-by: John Hubbard <jhubbard@...dia.com> thanks, -- John Hubbard NVIDIA > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 8bbcccbc5565..7a7a287818ad 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2941,80 +2941,6 @@ static inline vm_fault_t vmf_error(int err) > struct page *follow_page(struct vm_area_struct *vma, unsigned long address, > unsigned int foll_flags); > > -#define FOLL_WRITE 0x01 /* check pte is writable */ > -#define FOLL_TOUCH 0x02 /* mark page accessed */ > -#define FOLL_GET 0x04 /* do get_page on page */ > -#define FOLL_DUMP 0x08 /* give error on hole if it would be zero */ > -#define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */ > -#define FOLL_NOWAIT 0x20 /* if a disk transfer is needed, start the IO > - * and return without waiting upon it */ > -#define FOLL_NOFAULT 0x80 /* do not fault in pages */ > -#define FOLL_HWPOISON 0x100 /* check page is hwpoisoned */ > -#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */ > -#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */ > -#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */ > -#define FOLL_ANON 0x8000 /* don't do file mappings */ > -#define FOLL_LONGTERM 0x10000 /* mapping lifetime is indefinite: see below */ > -#define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */ > -#define FOLL_PIN 0x40000 /* pages must be released via unpin_user_page */ > -#define FOLL_FAST_ONLY 0x80000 /* gup_fast: prevent fall-back to slow gup */ > - > -/* > - * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each > - * other. Here is what they mean, and how to use them: > - * > - * FOLL_LONGTERM indicates that the page will be held for an indefinite time > - * period _often_ under userspace control. This is in contrast to > - * iov_iter_get_pages(), whose usages are transient. > - * > - * FIXME: For pages which are part of a filesystem, mappings are subject to the > - * lifetime enforced by the filesystem and we need guarantees that longterm > - * users like RDMA and V4L2 only establish mappings which coordinate usage with > - * the filesystem. Ideas for this coordination include revoking the longterm > - * pin, delaying writeback, bounce buffer page writeback, etc. As FS DAX was > - * added after the problem with filesystems was found FS DAX VMAs are > - * specifically failed. Filesystem pages are still subject to bugs and use of > - * FOLL_LONGTERM should be avoided on those pages. > - * > - * FIXME: Also NOTE that FOLL_LONGTERM is not supported in every GUP call. > - * Currently only get_user_pages() and get_user_pages_fast() support this flag > - * and calls to get_user_pages_[un]locked are specifically not allowed. This > - * is due to an incompatibility with the FS DAX check and > - * FAULT_FLAG_ALLOW_RETRY. > - * > - * In the CMA case: long term pins in a CMA region would unnecessarily fragment > - * that region. And so, CMA attempts to migrate the page before pinning, when > - * FOLL_LONGTERM is specified. > - * > - * FOLL_PIN indicates that a special kind of tracking (not just page->_refcount, > - * but an additional pin counting system) will be invoked. This is intended for > - * anything that gets a page reference and then touches page data (for example, > - * Direct IO). This lets the filesystem know that some non-file-system entity is > - * potentially changing the pages' data. In contrast to FOLL_GET (whose pages > - * are released via put_page()), FOLL_PIN pages must be released, ultimately, by > - * a call to unpin_user_page(). > - * > - * FOLL_PIN is similar to FOLL_GET: both of these pin pages. They use different > - * and separate refcounting mechanisms, however, and that means that each has > - * its own acquire and release mechanisms: > - * > - * FOLL_GET: get_user_pages*() to acquire, and put_page() to release. > - * > - * FOLL_PIN: pin_user_pages*() to acquire, and unpin_user_pages to release. > - * > - * FOLL_PIN and FOLL_GET are mutually exclusive for a given function call. > - * (The underlying pages may experience both FOLL_GET-based and FOLL_PIN-based > - * calls applied to them, and that's perfectly OK. This is a constraint on the > - * callers, not on the pages.) > - * > - * FOLL_PIN should be set internally by the pin_user_pages*() APIs, never > - * directly by the caller. That's in order to help avoid mismatches when > - * releasing pages: get_user_pages*() pages must be released via put_page(), > - * while pin_user_pages*() pages must be released via unpin_user_page(). > - * > - * Please see Documentation/core-api/pin_user_pages.rst for more information. > - */ > - > static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags) > { > if (vm_fault & VM_FAULT_OOM) > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 500e536796ca..0c80a5ad6e6a 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -1003,4 +1003,77 @@ enum fault_flag { > > typedef unsigned int __bitwise zap_flags_t; > > +/* > + * FOLL_PIN and FOLL_LONGTERM may be used in various combinations with each > + * other. Here is what they mean, and how to use them: > + * > + * FOLL_LONGTERM indicates that the page will be held for an indefinite time > + * period _often_ under userspace control. This is in contrast to > + * iov_iter_get_pages(), whose usages are transient. > + * > + * FIXME: For pages which are part of a filesystem, mappings are subject to the > + * lifetime enforced by the filesystem and we need guarantees that longterm > + * users like RDMA and V4L2 only establish mappings which coordinate usage with > + * the filesystem. Ideas for this coordination include revoking the longterm > + * pin, delaying writeback, bounce buffer page writeback, etc. As FS DAX was > + * added after the problem with filesystems was found FS DAX VMAs are > + * specifically failed. Filesystem pages are still subject to bugs and use of > + * FOLL_LONGTERM should be avoided on those pages. > + * > + * FIXME: Also NOTE that FOLL_LONGTERM is not supported in every GUP call. > + * Currently only get_user_pages() and get_user_pages_fast() support this flag > + * and calls to get_user_pages_[un]locked are specifically not allowed. This > + * is due to an incompatibility with the FS DAX check and > + * FAULT_FLAG_ALLOW_RETRY. > + * > + * In the CMA case: long term pins in a CMA region would unnecessarily fragment > + * that region. And so, CMA attempts to migrate the page before pinning, when > + * FOLL_LONGTERM is specified. > + * > + * FOLL_PIN indicates that a special kind of tracking (not just page->_refcount, > + * but an additional pin counting system) will be invoked. This is intended for > + * anything that gets a page reference and then touches page data (for example, > + * Direct IO). This lets the filesystem know that some non-file-system entity is > + * potentially changing the pages' data. In contrast to FOLL_GET (whose pages > + * are released via put_page()), FOLL_PIN pages must be released, ultimately, by > + * a call to unpin_user_page(). > + * > + * FOLL_PIN is similar to FOLL_GET: both of these pin pages. They use different > + * and separate refcounting mechanisms, however, and that means that each has > + * its own acquire and release mechanisms: > + * > + * FOLL_GET: get_user_pages*() to acquire, and put_page() to release. > + * > + * FOLL_PIN: pin_user_pages*() to acquire, and unpin_user_pages to release. > + * > + * FOLL_PIN and FOLL_GET are mutually exclusive for a given function call. > + * (The underlying pages may experience both FOLL_GET-based and FOLL_PIN-based > + * calls applied to them, and that's perfectly OK. This is a constraint on the > + * callers, not on the pages.) > + * > + * FOLL_PIN should be set internally by the pin_user_pages*() APIs, never > + * directly by the caller. That's in order to help avoid mismatches when > + * releasing pages: get_user_pages*() pages must be released via put_page(), > + * while pin_user_pages*() pages must be released via unpin_user_page(). > + * > + * Please see Documentation/core-api/pin_user_pages.rst for more information. > + */ > +#define FOLL_WRITE 0x01 /* check pte is writable */ > +#define FOLL_TOUCH 0x02 /* mark page accessed */ > +#define FOLL_GET 0x04 /* do get_page on page */ > +#define FOLL_DUMP 0x08 /* give error on hole if it would be zero */ > +#define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */ > +#define FOLL_NOWAIT 0x20 /* if a disk transfer is needed, start the IO > + * and return without waiting upon it */ > +#define FOLL_NOFAULT 0x80 /* do not fault in pages */ > +#define FOLL_HWPOISON 0x100 /* check page is hwpoisoned */ > +#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */ > +#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */ > +#define FOLL_REMOTE 0x2000 /* we are working on non-current tsk/mm */ > +#define FOLL_ANON 0x8000 /* don't do file mappings */ > +#define FOLL_LONGTERM 0x10000 /* mapping lifetime is indefinite: see below */ > +#define FOLL_SPLIT_PMD 0x20000 /* split huge pmd before returning */ > +#define FOLL_PIN 0x40000 /* pages must be released via unpin_user_page */ > +#define FOLL_FAST_ONLY 0x80000 /* gup_fast: prevent fall-back to slow gup */ > + > #endif /* _LINUX_MM_TYPES_H */ > >
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