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Manual Pages  — VOP_READ_PGCACHE

NAME

VOP_READ_PGCACHE – read a file, fast

CONTENTS

SYNOPSIS

#include <sys/param.h>
#include <sys/vnode.h>
#include <sys/uio.h>

int
VOP_READ_PGCACHE(struct vnode *vp, struct uio *uio, int ioflag, struct ucred *cred);

DESCRIPTION

This entry point reads the contents of a file. The intent is to provide the data from caches, which do not require expensive operations or any disk IO. For instance, if filesystem uses normal VM page cache and maintains v_object lifetime, it can use vn_read_from_obj(9) helper to return data from the resident vp->v_object pages.

The filesystem indicates support for the VOP_READ_PGCACHE on specific vnode by setting the VIRF_PGREAD flag in vp->v_irflag.

The function does not need to satisfy the whole request; it also might choose to not provide any data. In these cases, the uio must be advanced by the amount of read data, VOP_READ_PGCACHE should return EJUSTRETURN, and VFS would handle the rest of the read operation using the VOP_READ(9).

The VFS layer does the same deadlock avoidance for accessing userspace pages from VOP_READ_PGCACHE as for VOP_READ(9).

Vnode is not locked on the call entry and should not be locked on return. For a filesystem that requires vnode lock to return any data, it does not make sense to implement VOP_READ_PGCACHE (and set VIRF_PGREAD flag) since VFS arranges the call to VOP_READ(9) as needed.

The arguments are:
vp
  The vnode of the file.
uio
  The location of the data to be read.
ioflag
  Various flags, see VOP_READ(9) for the list.
cred
  The credentials of the caller.

VOP_READ_PGCACHE does not handle non-zero ioflag argument.

LOCKS

The file should be referenced on entry on entry and will still be referenced on exit. Rangelock covering the whole read range should be owned around the call.

RETURN VALUES

Zero is returned on success, when the whole request is satisfied, and no more data cannot be provided for it by any means. If more data can be returned, but VOP_READ_PGCACHE was unable to provide it, EJUSTRETURN must be returned. The uio records should be updated according to the partial operation done.

Otherwise an error code is returned, same as from VOP_READ(9)

SEE ALSO

uiomove(9), vnode(9), VOP_READ(9)
VOP_READ_PGCACHE (9) February 28, 2021
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